feat: implement gRPC storage plugin for metrics

[jacobmarble]

Which problem is this PR solving?

Resolves #4413

Short description of the changes

• define the metrics gRPC API in protos
• refactor: move two subpackages of plugin/storage/grpc to pkg/grpc
• implement the gRPC metrics plugin

A POC is implemented here: https://github.com/influxdata/influxdb-observability/pull/239

TODO

• [ ] initial review by maintainers
• [ ] add unit tests (new _test.go files with TODO comments)

[codecov[bot]]

Codecov Report

Patch coverage: 32.12% and project coverage change: -0.80 :warning:

Comparison is base (3021d31) 97.06% compared to head (61b0f86) 96.26%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4414      +/-   ##
==========================================
- Coverage   97.06%   96.26%   -0.80%     
==========================================
  Files         300      303       +3     
  Lines       17737    17955     +218     
==========================================
+ Hits        17217    17285      +68     
- Misses        417      558     +141     
- Partials      103      112       +9     

Flag	Coverage Δ
unittests	96.26% <32.12%> (-0.80%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
plugin/metrics/factory.go	88.63% <0.00%> (-11.37%)	:arrow_down:
plugin/metrics/grpc/factory.go	0.00% <0.00%> (ø)
plugin/metrics/grpc/options.go	0.00% <0.00%> (ø)
plugin/storage/grpc/grpc.go	0.00% <0.00%> (ø)
plugin/storage/grpc/shared/metrics.go	0.00% <0.00%> (ø)
plugin/storage/grpc/shared/plugin.go	0.00% <0.00%> (ø)
plugin/storage/grpc/shared/grpc_client.go	84.91% <50.00%> (-0.80%)	:arrow_down:
plugin/storage/grpc/shared/grpc_handler.go	74.11% <73.40%> (-0.36%)	:arrow_down:

... and 1 file with indirect coverage changes

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