BUILDS on #44342. Please review that one first.
INCREMENTAL DIFFS

Commit Message

Add gRPC streaming client for reverse tunnel reporting

Additional Description

Adds a bidirectional gRPC streaming client that pushes reverse tunnel connection state to a remote ReverseTunnelReportingService. This is a client implementation for the EventReporter introduced in the previous PR.

Protocol:

• On connect (and every reconnect) the client does a full state push of all known connections obtained from the reporter.
• Between connects the client sends incremental diffs on a periodic send timer. Empty messages serve as heartbeats.
• Each request carries an incrementing nonce. The server ACKs by echoing the nonce; a NACK carries an error_detail and triggers disconnect.
• If too many nonces remain unacked the client disconnects and reconnects, treating the server as dead or stuck.
• The server may adjust the send interval via report_interval in its ACK; the client floors it at 25ms to prevent tight loops.

Key components:

• GrpcClient: implements ReverseTunnelReporterClient and Grpc::AsyncStreamCallbacks. Manages stream lifecycle, nonce tracking, send/retry timers, and per-cluster stats.
• GrpcClientFactory: dynamically registered factory resolved by the EventReporter when constructing clients from config.
• GrpcClientConfig: parsed config struct with defaults (5s send interval, 5s retry, 5 max retries, 1M buffer).

Additional considerations:

• initialized_ guard prevents silent event queuing if cluster lookup or client creation fails during onServerInitialized().
• Buffer overflow while disconnected does not re-arm the retry timer.
• Nonce reset on every new stream to avoid stale ACK confusion.

Risk Level: Low

Testing

Unit tests

Request variables

Key	Value
ref	ac3e56d
sha	d568b1f
pr	44344
base-sha	94828ca
actor	@aakugan
message	reverse_tunnels: add gRPC streaming client for reverse tunnel reporting...
started	1775736158.192682
target-branch	main
trusted	false

Container image/s (as used in this CI run)

Envoy version (as used in this CI run)