Metrics: support expose metrics for plugins

[wonderflow]

Fixes #820

[codecov]

Codecov Report

Attention: Patch coverage is 16.07143% with 47 lines in your changes missing coverage. Please review.

Project coverage is 66.92%. Comparing base (0474e1d) to head (96b4b3d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
api/pkg/filtermanager/api_impl.go	3.22%	30 Missing ⚠️
api/pkg/filtermanager/config.go	31.57%	12 Missing and 1 partial ⚠️
api/pkg/plugins/plugins.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #827       +/-   ##
===========================================
- Coverage   85.44%   66.92%   -18.52%     
===========================================
  Files         141      101       -40     
  Lines        8407     4828     -3579     
===========================================
- Hits         7183     3231     -3952     
- Misses        969     1444      +475     
+ Partials      255      153      -102     

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[spacewander]

To test the behavior, we can define a new API like

htnn/api/plugins/tests/integration/dataplane/data_plane.go

Line 606 in 982b45c

func (dp *DataPlane) SetLogLevel(loggerName string, level string) error {

to fetches the metrics.

[wonderflow]

FYI, this PR currently blocked by: envoyproxy/envoy#37808