Migrate collector exporter self-observability from OpenCensus to OpenTelemetry

[dashpole]

Using OpenTelemetry for self-observability metrics is now stable in the collector: open-telemetry/opentelemetry-collector#9102. OpenCensus metrics are still supported on the prometheus endpoint: https://github.com/open-telemetry/opentelemetry-collector/blob/7ade1016cf138965685e533f60ec83892d26abc6/service/internal/proctelemetry/config.go#L212, but the OpenCensus bridge is not used if you are exporting with OTLP, which we might want to support in the future.

Our OpenCensus usage:

We define googlecloudmonitoring/point_count and googlecloudmonitoring/exemplar_attachments_dropped in exporter/collector/observability.go using OpenCensus. Migrating this to OpenTelemetry should be straightforward.

We also use ocgrpc in the collector exporter:

opentelemetry-operations-go/exporter/collector/config.go

Line 263 in fda999e

grpc.WithStatsHandler(&ocgrpc.ClientHandler{}),

. Migrating this to OTel may be tricky, as we would need to either migrate to otelgrpc, or to gRPC's upcoming self-observability metrics: https://github.com/grpc/proposal/blob/master/A66-otel-stats.md. This is likely to be a breaking change, which we will need to migrate with a featuregate.

[dashpole]

cc @braydonk @quentinmit