Change rpc.*.duration unit from ms to sec, clarify duration in case of streaming

[lmolkova]

Fixes #383

This PR

• renames rpc.client|server.duration to rpc.client|server.call.duration: this creates room for per-try metrics (see https://github.com/grpc/proposal/blob/master/A66-otel-stats.md#client-per-attempt-instruments)
• clarifies that metric and span measure full duration (full streaming call, full unary call with retries if done by the rpc fx) - this aligns with existing Java gRPC instrumentation and Google's gRPC metrics spec