HTTP and gRPC configuration: make RPC instrumentation into middleware

[jmacd]

Both gRPC and HTTP configuration automatically enable standard OpenTelemetry instrumentation packages. The use of RPC-level instrumentation should be optional and configurable.

Now that we have middleware extensions, we are able to configure alternative OpenTelemetry instrumentation packages via middleware hooks. We can use a feature flag to migrate from built-in instrumentation (always enabled) to optional configuration via built-in and optional extensions. This will allow experimentation with RPC instrumentation packages and allow RPC instrumentation to be disabled.

See #13228 for a similar discussion about compression middleware for HTTP configuration.