feat(storage): log effective gRPC transport and channel connection readiness - #16478

When DirectPath over Interconnect is enabled, applications cannot easily verify whether it took effect. Options such as EndpointOption or UniverseDomainOption silently override the feature, and there is no built-in signal indicating how long the initial channel takes to connect. This is a follow-up to #16408.

To resolve this, this PR adds google/cloud/storage/internal/grpc/channel_telemetry.{h,cc} to introduce the following functionality:

• Transport Classification: TransportType and DetectTransportType() categorize the effective endpoint as CloudPath, DirectPath, or DirectPathInterconnect.
• Configuration Logging: LogChannelConfiguration(), invoked during CreateDecoratedStubs(), logs an INFO message detailing the transport type. It emits a WARNING if DirectPathXdsOverInterconnectOption is configured but not requested by the effective endpoint.
• Connection Telemetry: StartChannelTelemetry(), invoked during CreateStorageStub(), tracks AsyncWaitConnectionReady() on the initial channel to log connection duration. Because connection attempts are already initiated at this point via GrpcChannelRefresh, this tracking is purely observational and does not alter behavior.

Design Considerations:

• Channel Selection: Telemetry is tracked only for channel 0. Since certain VM environments spawn hundreds of channels, this approach aligns with the performance trade-offs noted in GrpcChannelRefresh::Refresh().
• Log Severity for Failures: Connection readiness failures are logged at INFO rather than WARNING. A completion queue shutting down prior to connection completes with kDeadlineExceeded, which cannot be reliably distinguished from a true connection timeout. Logging WARNING in these cases would cause false positives in short-lived operations, so WARNING is reserved strictly for clear configuration mismatches.

Note: The google-c2p resolver might still fall back to CloudPath internally. Because this internal fallback cannot be detected by the client library, DetectTransportType() strictly reflects the requested transport type.