feat(inputs/memcached): Support client TLS origination to memcached

[LINKIWI]

Required for all PRs:

• Updated associated README.md.
• Wrote appropriate unit tests.
• Pull request title or commits are in conventional commit format

This change adds support for client TLS origination to memcached, leveraging the common TLS plugin library in plugins/common/tls. I also added toml struct tags to make the TOML parsing behavior explicit, which seems to be more in line with other plugins as well.

Test and verification:

Collection with server TLS enabled, using the memcached test fixture certificates in t/server_crt.pem et al.:

# telegraf.conf
[[inputs.memcached]]
    enable_tls = true
    tls_ca = "/path/to/memcached/t/cacert.pem"
    tls_cert = "/path/to/memcached/t/client_crt.pem"
    tls_key = "/path/to/memcached/t/client_key.pem"
    tls_server_name = "test.com"

# Compiled with ./configure --enable-tls
$ ./memcached --enable-ssl --extended ssl_chain_cert=t/server_crt.pem,ssl_key=t/server_key.pem,ssl_ca_cert=t/cacert.pem -vv
slab class   1: chunk size        96 perslab   10922
slab class   2: chunk size       120 perslab    8738
slab class   3: chunk size       152 perslab    6898
slab class   4: chunk size       192 perslab    5461
slab class   5: chunk size       240 perslab    4369
slab class   6: chunk size       304 perslab    3449
slab class   7: chunk size       384 perslab    2730
slab class   8: chunk size       480 perslab    2184
slab class   9: chunk size       600 perslab    1747
slab class  10: chunk size       752 perslab    1394
slab class  11: chunk size       944 perslab    1110
slab class  12: chunk size      1184 perslab     885
slab class  13: chunk size      1480 perslab     708
slab class  14: chunk size      1856 perslab     564
slab class  15: chunk size      2320 perslab     451
slab class  16: chunk size      2904 perslab     361
slab class  17: chunk size      3632 perslab     288
slab class  18: chunk size      4544 perslab     230
slab class  19: chunk size      5680 perslab     184
slab class  20: chunk size      7104 perslab     147
slab class  21: chunk size      8880 perslab     118
slab class  22: chunk size     11104 perslab      94
slab class  23: chunk size     13880 perslab      75
slab class  24: chunk size     17352 perslab      60
slab class  25: chunk size     21696 perslab      48
slab class  26: chunk size     27120 perslab      38
slab class  27: chunk size     33904 perslab      30
slab class  28: chunk size     42384 perslab      24
slab class  29: chunk size     52984 perslab      19
slab class  30: chunk size     66232 perslab      15
slab class  31: chunk size     82792 perslab      12
slab class  32: chunk size    103496 perslab      10
slab class  33: chunk size    129376 perslab       8
slab class  34: chunk size    161720 perslab       6
slab class  35: chunk size    202152 perslab       5
slab class  36: chunk size    252696 perslab       4
slab class  37: chunk size    315872 perslab       3
slab class  38: chunk size    394840 perslab       2
slab class  39: chunk size    524288 perslab       2
<22 server listening (auto-negotiate)
<23 server listening (auto-negotiate)
<24 new auto-negotiating client connection
24: Client using the ascii protocol
<24 stats
<24 connection closed.

$ ./telegraf --config telegraf.conf --test
...
> memcached,server=:11211 accepting_conns=1i,auth_cmds=0i,auth_errors=0i,bytes=0i,bytes_read=7i,bytes_written=0i,cas_badval=0i,cas_hits=0i,cas_misses=0i,cmd_flush=0i,cmd_get=0i,cmd_set=0i,cmd_touch=0i,conn_yields=0i,connection_structures=3i,curr_connections=2i,curr_items=0i,decr_hits=0i,decr_misses=0i,delete_hits=0i,delete_misses=0i,evicted_unfetched=0i,evictions=0i,expired_unfetched=0i,get_hits=0i,get_misses=0i,hash_bytes=524288i,hash_is_expanding=0i,hash_power_level=16i,incr_hits=0i,incr_misses=0i,limit_maxbytes=67108864i,listen_disabled_num=0i,reclaimed=0i,threads=4i,total_connections=3i,total_items=0i,touch_hits=0i,touch_misses=0i,uptime=21i 1644773022000000000

Collection with server TLS disabled, to verify backwards compatibility:

# telegraf.conf
[[inputs.memcached]]

$ ./memcached -vv
slab class   1: chunk size        96 perslab   10922
slab class   2: chunk size       120 perslab    8738
slab class   3: chunk size       152 perslab    6898
slab class   4: chunk size       192 perslab    5461
slab class   5: chunk size       240 perslab    4369
slab class   6: chunk size       304 perslab    3449
slab class   7: chunk size       384 perslab    2730
slab class   8: chunk size       480 perslab    2184
slab class   9: chunk size       600 perslab    1747
slab class  10: chunk size       752 perslab    1394
slab class  11: chunk size       944 perslab    1110
slab class  12: chunk size      1184 perslab     885
slab class  13: chunk size      1480 perslab     708
slab class  14: chunk size      1856 perslab     564
slab class  15: chunk size      2320 perslab     451
slab class  16: chunk size      2904 perslab     361
slab class  17: chunk size      3632 perslab     288
slab class  18: chunk size      4544 perslab     230
slab class  19: chunk size      5680 perslab     184
slab class  20: chunk size      7104 perslab     147
slab class  21: chunk size      8880 perslab     118
slab class  22: chunk size     11104 perslab      94
slab class  23: chunk size     13880 perslab      75
slab class  24: chunk size     17352 perslab      60
slab class  25: chunk size     21696 perslab      48
slab class  26: chunk size     27120 perslab      38
slab class  27: chunk size     33904 perslab      30
slab class  28: chunk size     42384 perslab      24
slab class  29: chunk size     52984 perslab      19
slab class  30: chunk size     66232 perslab      15
slab class  31: chunk size     82792 perslab      12
slab class  32: chunk size    103496 perslab      10
slab class  33: chunk size    129376 perslab       8
slab class  34: chunk size    161720 perslab       6
slab class  35: chunk size    202152 perslab       5
slab class  36: chunk size    252696 perslab       4
slab class  37: chunk size    315872 perslab       3
slab class  38: chunk size    394840 perslab       2
slab class  39: chunk size    524288 perslab       2
<22 server listening (auto-negotiate)
<23 server listening (auto-negotiate)
<24 new auto-negotiating client connection
24: Client using the ascii protocol
<24 stats
<24 connection closed.

$ ./telegraf --config telegraf.conf --test
...
> memcached,server=:11211 accepting_conns=1i,auth_cmds=0i,auth_errors=0i,bytes=0i,bytes_read=7i,bytes_written=0i,cas_badval=0i,cas_hits=0i,cas_misses=0i,cmd_flush=0i,cmd_get=0i,cmd_set=0i,cmd_touch=0i,conn_yields=0i,connection_structures=3i,curr_connections=2i,curr_items=0i,decr_hits=0i,decr_misses=0i,delete_hits=0i,delete_misses=0i,evicted_unfetched=0i,evictions=0i,expired_unfetched=0i,get_hits=0i,get_misses=0i,hash_bytes=524288i,hash_is_expanding=0i,hash_power_level=16i,incr_hits=0i,incr_misses=0i,limit_maxbytes=67108864i,listen_disabled_num=0i,reclaimed=0i,threads=4i,total_connections=3i,total_items=0i,touch_hits=0i,touch_misses=0i,uptime=11i 1644773145000000000

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[srebhan]

One (maybe off-topic) question, could this eliminate the need for https://github.com/influxdata/telegraf/tree/master/plugins/common/proxy?

[LINKIWI]

Thanks for the review. Looks like the common/proxy package is mostly just a wrapper around x/net/proxy to generate a SOCKS5 or HTTP Dialer from a Telegraf TOML config. I haven't touched that part of the codebase before but I imagine it's useful to keep around.

[srebhan]

The reason I'm asking is that I'm a bit hesitant to have both ways in the code-base as it calls for confusion. So ideally you should use common/proxy or we should remove common/proxy altogether (if it replicates an upstream package). Do you think that's possible?

[LINKIWI]

Oh, I see. The usage of golang.org/x/net/proxy in this PR is just to type the dialer variable against the proxy.Dialer interface type. There isn't any actual usage of proxy logic.

If you like, I can restructure the logic so that the golang.org/x/net/proxy import is no longer needed. But there would be more code duplication. Something like

plaintextDialer := ...
var tlsDialer *tls.Dialer

if enable tls {
  tlsCfg, err := m.ClientConfig.TLSConfig()
  ...
  tlsDialer = ...
}

...

// following repeated twice, for each of unix socket and TCP address endpoints
if enable tls {
  conn, err := tlsDialer.Dial(...)
  ...
} else {
  conn, err := plaintextDialer.Dial(...)
  ...
}

[srebhan]

Looks good to me. Thanks @LINKIWI!