Fix external loggers respecting configured format

[Rohithmatham12]

What this changes

• Routes gRPC package-level logs through the configured Cortex go-kit logger so transport errors respect server.log_format.
• Replaces the side-effect automaxprocs import with explicit maxprocs.Set(...) after Cortex initializes logging, so maxprocs messages also use the configured format.
• Preserves gRPC default severity behavior: errors only unless GRPC_GO_LOG_SEVERITY_LEVEL asks for warning/info.
• Adds focused tests for gRPC and automaxprocs log output through the configured JSON logger.

Fixes #7697.

Testing

• GOCACHE=/private/tmp/cortex-go-cache GOMODCACHE=/private/tmp/cortex-go-mod-cache go test ./pkg/util/log
• GOCACHE=/private/tmp/cortex-go-cache GOMODCACHE=/private/tmp/cortex-go-mod-cache go test ./cmd/cortex