balancer: Move metrics recorder from BuildOptions to ClientConn

[arjan-bal]

This PR changes the way LB policies obtain a metrics recorder from the channel. Earlier LB policies obtained it through the BuildOptions. Now the LB policies need to call the MetricsRecorder method on the ClientConn.

Why

When LB policies manage child policies, they need to correctly propagate the BuildOptions. If they fail to pass down the MetricsRecorder in the BuildOptions, it will read to nil pointer deferences when the child tries to record metrics. There is no way for gRPC to enforce LB polices propagate the MetricsRecorder. #8026 is open to enforce LB policies embed an existing ClientConn object. Using a method on ClientConn ensures that the MetricsRecorder can be obtained from the channel, unless explicitly intercepted by an LB policy. The MetricsRecorder field in the BuildOptions was added in the past 6 months, so we don't expect too many breakages in external code.

RELEASE NOTES:

• balancer: Implementors of custom LB policies that record metrics must use the new MetricsRecorder method on Balancer.ClientConn instead of the removed Balancer.BuildOptions.MetricsRecorder field to obtain a metrics recorder.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.23%. Comparing base (3e27c17) to head (184db85).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8027      +/-   ##
==========================================
- Coverage   82.47%   82.23%   -0.24%     
==========================================
  Files         387      387              
  Lines       39037    39040       +3     
==========================================
- Hits        32194    32106      -88     
- Misses       5530     5602      +72     
- Partials     1313     1332      +19     

Files with missing lines	Coverage Δ
balancer/balancer.go	96.29% <ø> (ø)
balancer/pickfirst/pickfirstleaf/pickfirstleaf.go	86.70% <100.00%> (-0.58%)	⬇️
balancer/rls/balancer.go	85.30% <100.00%> (-0.79%)	⬇️
balancer/weightedroundrobin/balancer.go	84.22% <100.00%> (+0.84%)	⬆️
balancer_wrapper.go	86.34% <100.00%> (-0.99%)	⬇️
internal/testutils/balancer.go	82.53% <100.00%> (+0.18%)	⬆️

... and 25 files with indirect coverage changes

[arjan-bal]

Merged a PR to avoid breakages in grpc-gcp-go: GoogleCloudPlatform/grpc-gcp-go#91
Waiting on release of https://github.com/GoogleCloudPlatform/grpc-gcp-go.