fix: End BIDI gRPC transactions on response send instead of stream close

[nlakhera]

Overview

For bidirectional streaming gRPC calls, transactions now end when the first response is sent rather than when the stream closes. This provides accurate transaction durations (typically 2-10ms) instead of measuring the entire stream lifecycle (30-180 seconds).

Changes:

• ServerImpl_Instrumentation: Detect BIDI streaming and mark streams
• ServerStream_Instrumentation: Instrument writeMessage() to expire token after first response is sent
• Only affects BIDI_STREAMING, other gRPC types unchanged

Fixes issue where transaction duration included stream idle time and upstream service latency, making performance monitoring ineffective.

Related: Internal discussion on gRPC transaction instrumentation

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