[BUG] OTLP gRPC metrics exporter crashes on startup — GrpcSenderProvider missing from classpath

[Gezi-lzq]

When configuring the OTLP metrics exporter with protocol=grpc, the broker crashes on startup with:

java.lang.IllegalStateException: No GrpcSenderProvider found on classpath.
Please add dependency on opentelemetry-exporter-sender-okhttp or opentelemetry-exporter-sender-grpc-upstream

PR #3124 added opentelemetry-exporter-sender-jdk and excluded opentelemetry-exporter-sender-okhttp from automq-metrics. sender-jdk only provides HttpSenderProvider via SPI — it does not provide GrpcSenderProvider. The only module that registers GrpcSenderProvider is sender-okhttp, which is explicitly excluded.

The OTLPProtocol enum still declares GRPC and OTLPMetricsExporter has a case GRPC branch that compiles (the API classes exist in exporter-otlp), but fails at runtime because no SPI implementation is available.