fix: defer OnStreamRecv so recv error and bytes are observed correctly

[steve-weiland]

Problem

streamMetricsInterceptor.Recv records its metric before calling the downstream handler, so err is always nil and proto.Size(msg) measures an empty message, every stream-recv metric reports err == nil and bytes == 0.

Send already defers correctly; Recv doesn't.

Fix

Defer OnStreamRecv so it runs after the handler returns, mirroring Send. Adds TestStreamMetricsRecvObservedAfterHandler, which fails on the old ordering and passes with the fix.

No behavioral change beyond metric accuracy.

[CLAassistant]

All committers have signed the CLA.

[steve-weiland]

Bumping this gently.