transport: Increment metrics only when the stream is active (#8573)

transport: Increment metrics only when the stream is active (#8573)

Fixes: https://github.com/grpc/grpc-go/issues/8529
This PR fixes to increment metrics only when the stream is active which
is found in the activeStreams map.

#### as-is
- The deleteStream was incrementing channelz metrics every time it was
called, even when stream was already removed from activeStreams or not
exists in activeStreams.

#### to-be
- Added check to ensure metrics are only incremented once when a stream
is actually removed from activeStreams.

RELEASE NOTES:
* server: Fix a bug that caused overcounting of channelz metrics for
successful and failed streams.