Add gRPC communication failure metrics for worker channel diagnostics

[liliankasem]

During investigation of ICM 699060688, we identified a gap in our ability to proactively detect transient gRPC communication failures between the Functions Host and language workers. Currently, when gRPC communication fails (connection refused, timeout, etc.), we lack metrics to aggregate and alert on these failures.

Background:

• SLA site ping failure occurred due to transient gRPC communication issue with Java 11 worker
• Issue self-mitigated but we had no visibility into failure frequency
• Similar incidents could occur without detection until they impact customer-facing metrics

Proposed Changes:

1. Add new metric constants to src/WebJobs.Script/Diagnostics/MetricEventNames.cs:

// gRPC communication metrics
public const string WorkerGrpcConnectionFailed = "{0}worker.grpc.connection.failed";
public const string WorkerGrpcMessageSendFailed = "{0}worker.grpc.message.send.failed";
public const string WorkerGrpcInitTimeout = "{0}worker.grpc.init.timeout";
public const string WorkerGrpcTransientError = "{0}worker.grpc.transient.error";

2. Emit metrics in src/WebJobs.Script.Grpc/Channel/GrpcWorkerChannel.cs:
   • HandleWorkerStartStreamError → emit WorkerGrpcConnectionFailed
   • HandleWorkerInitError → emit WorkerGrpcInitTimeout (when TimeoutException)
   • SendStreamingMessageAsync failure path → emit WorkerGrpcMessageSendFailed

[AartBluestoke]

Azure Logic apps with service bus triggers can have silent failures with wierd retries when the service bus condition glitches- I've had double triggers where one of the triggers falls to receipt the message. The support ticket closed with a "as designed" resolution, even though there were no log messages anywhere to indicate what had happened.