fix: Add reset-mode flag for CSV parser

[srebhan]

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

resolves #11257, resolves #10678

This PR adds a flag to allow the CSV parser to be reset on every call to Parse. This is required as, dependent on the input processing, the CSV parser needs to either keep track of the state (e.g. when parsing linewise) or needs to drop the state (e.g. when reading whole files). In the first case, you should set csv_reset_mode = "none" (default), while in the second case you should set csv_reset_mode = "always".

Please note: Reset-mode "always" is ignored for ParseLine (which is not used in our codebase) as this implicitly requires keeping track of the state if used with skipped-rows, headers and/or metadata.

[JackSharebourg]

@srebhan tanks for looking into this.
With csv_reset_mode = "always" the header of a new csv does not throw an error anymore, but it is also not read correctly. All csv's are parsed with the header information of the first read csv. See the example below. First cpu.csv is send, then as second file weather.csv. But as you can see in output weather.csv has the field keys given by cpu.csv.

test.conf

[[inputs.mqtt_consumer]]
  servers = ["tcp://127.0.0.1:1883"]

  topics = [
    "foo/#",
  ]
  topic_tag = ""

  data_format = "csv"
  csv_header_row_count = 1
  csv_timestamp_column = "time"
  csv_timestamp_format = "unix_ms"
  csv_skip_values = [""]
  csv_reset_mode = "always"

[[outputs.file]]
  files = ["stdout"]
  data_format = "influx"

[agent]
  omit_hostname = true

cpu.csv

time,measurement,cpu,time_user,time_system,time_idle
1645884455653,cpu,cpu0,42,42,42

weather.csv

time,outlook,temperature,humidity,windy,play
1645884455657,overcast,hot,high,FALSE,yes

output

mqtt_consumer cpu="cpu0",time_user=42i,time_system=42i,time_idle=42i,measurement="cpu" 1645884455653000000
mqtt_consumer measurement="overcast",cpu="hot",time_user="high",time_system=false,time_idle="yes" 1645884455657000000

[telegraf-tiger]

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armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz
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i386.deb	ppc64le.rpm	freebsd_i386.tar.gz
mips.deb	riscv64.rpm	linux_amd64.tar.gz
mipsel.deb	s390x.rpm	linux_arm64.tar.gz
ppc64el.deb	x86_64.rpm	linux_armel.tar.gz
riscv64.deb		linux_armhf.tar.gz
s390x.deb		linux_i386.tar.gz
		linux_mips.tar.gz
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		static_linux_amd64.tar.gz

[srebhan]

Can you please try again?

[JackSharebourg]

Works for me