Support standard gRPC metrics as per proposal A66

[dhowden]

Problem Statement

There is a gRPC proposal for standardising OpenTelemetry metrics (A66 https://github.com/grpc/proposal/blob/master/A66-otel-stats.md) linked from here: https://grpc.io/docs/guides/opentelemetry-metrics/#server-instruments.

The recommended gRPC integration (go.opentelemetry.io/contrib/instrumentation/google.golang.org/grpc/otelgrpc.ServerHandler) does not follow this standard:

• similar metrics, but with small differences in naming (rpc, not grpc).
• completely missing "call.started" count.

Proposed Solution

Implement the metrics proposed in A66.

Alternative Solution

Document the code to detail that it follows some other standard (and point to that).