stats/otel: add backend service attribute to weighted round robin metrics and add subchannel metrics

[mbissa]

Addresses :

• https://github.com/grpc/proposal/blob/master/A94-subchannel-otel-metrics.md and
• adding backend service label to weighted round robin metrics as defined in https://github.com/grpc/proposal/blob/master/A89-backend-service-metric-label.md

This PR makes available backend service (cluster name) label which is decided in clusterimpl (since we are pre-A75). It is added to weighted round robin per call metrics.
Also, this PR adds sub channel metrics which will eventually replace the pick first metrics.

RELEASE NOTES:

• stats/otel: add backend service label to wrr metrics and add subchannel metrics to eventually replace the pickfirst metrics

[codecov]

Codecov Report

❌ Patch coverage is 94.11765% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.19%. Comparing base (749af0c) to head (f3860ad).

Files with missing lines	Patch %	Lines
internal/testutils/stats/test_metrics_recorder.go	80.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8701      +/-   ##
==========================================
- Coverage   83.30%   83.19%   -0.12%     
==========================================
  Files         419      419              
  Lines       32450    32515      +65     
==========================================
+ Hits        27033    27050      +17     
- Misses       4039     4060      +21     
- Partials     1378     1405      +27     

Files with missing lines	Coverage Δ
balancer/weightedroundrobin/balancer.go	85.87% <100.00%> (+0.32%)	⬆️
balancer/weightedroundrobin/scheduler.go	96.22% <100.00%> (ø)
clientconn.go	90.47% <100.00%> (-0.28%)	⬇️
internal/internal.go	100.00% <ø> (ø)
internal/transport/http2_client.go	92.23% <100.00%> (-0.68%)	⬇️
internal/transport/transport.go	88.70% <ø> (-2.16%)	⬇️
internal/xds/balancer/clusterimpl/clusterimpl.go	83.79% <100.00%> (+0.07%)	⬆️
internal/xds/xds.go	81.08% <100.00%> (+6.08%)	⬆️
resolver/resolver.go	100.00% <100.00%> (ø)
internal/testutils/stats/test_metrics_recorder.go	79.48% <80.00%> (+5.87%)	⬆️

... and 24 files with indirect coverage changes

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[easwars]

This PR seems to address two things which are not directly related to each other:

• subchannel metrics
• adding the backend service label to WRR metrics

Can this be split into two PRs instead?