When testing CSM Observability, we discovered that the c++ xds interop client is not sending any payload with the `UnaryCall` RPCs so most of the metrics will have a value of 0.
Adding a payload to the xds interop client here.
We need this fix so that we can verify that the metrics are recording the right number of bytes being sent / received. So we need a non-trivial payload to be sent with the `UnaryCall` RPC between the xds interop client and server.
Closes#35545
COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35545 from stanley-cheung:xds-client-payload 11be4e6f4f
PiperOrigin-RevId: 601596246
This pull request adds another hook service on the maintenance server.
This will enable clients to gradually migrate from the standalone hook
server.
Changes:
1. Hook service can now be used separately.
2. Copied latest protos and updated the hook service to new API.
3. Added the hook service to the maintenance server.
Stanley Cheung