add prometheus metrics support

[songjiaxun]

Description

Add Prometheus metrics support. Add a new flag prometheus-port for users to specify prometheus metrics endpoint port.

Link to the issue in case of a bug fix.

NA

Testing details

1. Manual - Done

• Build the code using the command go run tools/build_gcsfuse/main.go . . v3.0.0.
• Use the following config file:

monitoring:
  prometheus-port: "8080"

• Mount a bucket: ./bin/gcsfuse --config-file <config_file_path> <bucket-name> <mount-path>.
• Pull the metrics: curl http://localhost:8080/metrics
• See the output example in the docs/metrics.md.

2. Unit tests - Done
3. Integration tests - NA
4. Performance tests

[sethiay]

Though this should be lightweight but we should do run perf testing of GCSfuse with promotheus server running and also ensure that this doesn't take extra memory and CPU

[songjiaxun]

Hi @sethiay, thanks for the review! I updated the PR to move the mount flag to config file. Do you happen to have a guidance to do the GCSFuse perf test and memory/CPU resource tests?

[sethiay]

Hey @songjiaxun We have presubmit performance tests (I have enabled for this PR with the labels) that do some basic performance testing without capturing CPU and memory. That is good to ensure that performance doesn't degrade incase some business logic is changed or dependencies are upgraded. However, this change requires more testing and monitoring CPU and memory, so I recommend running all the tests pipeline that we have and monitor the memory, CPU and performance numbers for the tests.

[codecov]

Codecov Report

Attention: Patch coverage is 8.10811% with 34 lines in your changes are missing coverage. Please review.

Project coverage is 61.09%. Comparing base (89392be) to head (1afeaac).
Report is 2 commits behind head on master.

Files	Patch %	Lines
internal/monitor/exporter.go	0.00%	31 Missing ⚠️
main.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1859      +/-   ##
==========================================
- Coverage   61.27%   61.09%   -0.19%     
==========================================
  Files         130      130              
  Lines       12458    12495      +37     
==========================================
  Hits         7634     7634              
- Misses       4481     4517      +36     
- Partials      343      344       +1     

Flag	Coverage Δ
unittests	61.09% <8.10%> (-0.19%)	⬇️

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