stats: change non-standard units to annotations

[arjan-bal]

gRPC Go has been specifying units as non-standard values such as call and endpoint instead of using annotations like {call}. Based on the metric naming conventions anything enclosed in curly braces is an annotation and in the absence of a prefix, the unit is 1. The gRPC docs also mention that non-standard units need to be enclosed in curly braces: https://grpc.io/docs/guides/opentelemetry-metrics/#per-call-metrics.

This bug was caught due to a regression in the otel prometheus exporter: #8479

RELEASE NOTES:

• stats: fixed metric unit formatting by enclosing non-standard units like call and endpoint in curly braces to comply with UCUM and gRPC OpenTelemetry guidelines.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.35%. Comparing base (ac13172) to head (9b04f93).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8481   +/-   ##
=======================================
  Coverage   82.35%   82.35%           
=======================================
  Files         413      413           
  Lines       40518    40518           
=======================================
+ Hits        33368    33369    +1     
+ Misses       5781     5779    -2     
- Partials     1369     1370    +1     

Files with missing lines	Coverage Δ
balancer/pickfirst/pickfirstleaf/pickfirstleaf.go	86.49% <ø> (-0.59%)	⬇️
balancer/rls/balancer.go	85.26% <ø> (ø)
balancer/weightedroundrobin/balancer.go	83.71% <ø> (ø)
stats/opentelemetry/client_metrics.go	85.08% <100.00%> (ø)
...tats/opentelemetry/internal/testutils/testutils.go	94.69% <100.00%> (ø)
stats/opentelemetry/server_metrics.go	87.26% <100.00%> (ø)
xds/internal/xdsclient/clientimpl.go	82.52% <ø> (ø)

... and 16 files with indirect coverage changes

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