add ewma balancer

[redynasc]

What this PR does / why we need it:

ewma is a different balancing implementation that will generate a weight for every backend IP based on the last server response time, basically it tries to dispatch more requests to the backends that reply faster, supposing that they are less loaded.

fix #1996

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible?

[membphis]

real request for testing

[membphis]

@redynasc please take a look at the output of CI: https://github.com/apache/apisix/pull/2001/checks?check_run_id=952937828

+ luacheck -q apisix
Checking apisix/balancer/ewma.lua                 7 warnings

    apisix/balancer/ewma.lua:146:16: unused function calculate_slow_start_ewma
    apisix/balancer/ewma.lua:176:11: unused variable k
    apisix/balancer/ewma.lua:176:13: unused variable _
make: *** [lint] Error 1
    apisix/balancer/ewma.lua:192:21: variable ewma_score is never accessed
    apisix/balancer/ewma.lua:196:101: line is too long (102 > 100)
    apisix/balancer/ewma.lua:206:5: right side of assignment has less values than left side expects
    apisix/balancer/ewma.lua:225:101: line is too long (102 > 100)
+ exit 1

[membphis]

@redynasc if we have fixed them, we can click the resolve ... button by ourselves.

[membphis]

it seems much better now.

[moonming]

@membphis please take a look, and I think we need more test cases, WDYT?

[moonming]

@redynasc For the code of other projects, we cannot add ASF header, please take a look at https://github.com/apache/skywalking/blob/master/LICENSE#L204, which is the correct way to deal with license issue.

[membphis]

@redynasc we need more test cases

[moonming]

@redynasc ping

[moonming]

@redynasc any update?

[redynasc]

add one new test case，It is difficult to add more test items, any suggestions？

[membphis]

@redynasc it seems that the new test case is unstable. we need to fix this before we merge this PR

#   Failed test 'TEST 3: about frequency - response_body - response is expected (repeated req 0, req 0)'
#   at /home/runner/work/apisix/apisix/test-nginx/lib/Test/Nginx/Socket.pm line 1589.
#          got: '{"count":1,"port":"1980"}
# '
#     expected: '{"count":1,"port":"1981"}
# '
# Looks like you failed 1 test of 9.
t/node/ewma.t ............................ 

[redynasc]

@redynasc it seems that the new test case is unstable. we need to fix this before we merge this PR

#   Failed test 'TEST 3: about frequency - response_body - response is expected (repeated req 0, req 0)'
#   at /home/runner/work/apisix/apisix/test-nginx/lib/Test/Nginx/Socket.pm line 1589.
#          got: '{"count":1,"port":"1980"}
# '
#     expected: '{"count":1,"port":"1981"}
# '
# Looks like you failed 1 test of 9.
t/node/ewma.t ............................ 

Do I need to care about the following errors in test?
2020/08/24 03:45:37 [error] 32670#32670: init_by_lua error: /usr/share/lua/5.1/tinyyaml.lua:698: bad argument #1 to 'sfind' (string expected, got nil)

[moonming]

the license issue is not fixed, which is MUST fixed before next release.

[redynasc]

the license issue is not fixed, which is MUST fixed before next release.

I added the "Apache ApiSix Subcomponent" section to the license file
I don't know how to solve this problem and need help

[moonming]

I will help to fix license issue after this PR merged. Thanks, Ming Wen Twitter: _WenMing redynasc <notifications@github.com> 于2020年8月27日周四 上午10:52写道：

…

the license issue is not fixed, which is MUST fixed before next release. I added the "Apache ApiSix Subcomponent" section to the license file I don't know how to solve this problem and need help — You are receiving this because you commented. Reply to this email directly, view it on GitHub <#2001 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AGJZBKYKTU64GZEOPGIPFILSCXC6NANCNFSM4PVUIBEA> .

[redynasc]

font{ line-height: 1.6; } ul,ol{ padding-left: 20px; list-style-position: inside; } Thank you very much. ifonly.dgg ifonly.dgg@gmail.com 签名由 网易邮箱大师 定制 On 8/27/2020 11:56，Wen Ming<notifications@github.com> wrote： I will help to fix license issue after this PR merged. Thanks, Ming Wen Twitter: _WenMing redynasc <notifications@github.com> 于2020年8月27日周四 上午10:52写道：

the license issue is not fixed, which is MUST fixed before next release. I added the "Apache ApiSix Subcomponent" section to the license file I don't know how to solve this problem and need help — You are receiving this because you commented. Reply to this email directly, view it on GitHub <#2001 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AGJZBKYKTU64GZEOPGIPFILSCXC6NANCNFSM4PVUIBEA> .

—You are receiving this because you were mentioned.Reply to this email directly, view it on GitHub, or unsubscribe.

fix license later

[membphis]

@redynasc merged, many thx