Add Counter Output Operator

[schmikei]

Description of Changes

Adds a new output operator: count_output

The purpose of this operator is to count the number of entries processed over an interval.

Proposed config:

pipeline:
- type: count_output
   path: /optional/path/to/output/file # otherwise stdout
   duration: 1m # duration of the output 

Please check that the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• Add a changelog entry (for non-trivial bug fixes / features)
• CI passes

[codecov]

Codecov Report

Merging #570 (65fa531) into master (3b23e5c) will decrease coverage by 1.80%.
The diff coverage is 86.29%.

@@            Coverage Diff             @@
##           master     #570      +/-   ##
==========================================
- Coverage   76.69%   74.88%   -1.80%     
==========================================
  Files         132      133       +1     
  Lines        8308    10076    +1768     
==========================================
+ Hits         6371     7545    +1174     
- Misses       1473     2065     +592     
- Partials      464      466       +2     

Impacted Files	Coverage Δ
operator/buffer/buffer.go	75.86% <ø> (-1.92%)	⬇️
operator/buffer/memory.go	71.60% <ø> (-3.74%)	⬇️
...builtin/input/aws/cloudwatch/cloudwatch_persist.go	75.00% <ø> (-6.82%)	⬇️
operator/builtin/input/azure/event_hub.go	0.00% <0.00%> (ø)
operator/builtin/input/azure/event_hub_config.go	90.91% <ø> (-1.09%)	⬇️
operator/builtin/input/file/finder.go	100.00% <ø> (ø)
operator/builtin/input/file/reader.go	53.05% <0.00%> (-5.73%)	⬇️
operator/builtin/input/http/config.go	97.56% <ø> (-0.29%)	⬇️
operator/builtin/input/journald/journald.go	58.76% <ø> (-1.83%)	⬇️
operator/builtin/input/udp/udp.go	73.49% <ø> (-1.51%)	⬇️
... and 157 more

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[schmikei]

May look into adding https://pkg.go.dev/expvar as another flag and thats what we publish. Not sure if its necessary on this first pass.

[cpheps]

Overall looks good just come comments