Fixes https://emqx.atlassian.net/browse/EMQX-9603
Rather than relying on a JWT worker to produce and refresh tokens, we
could just produce then on demand when pushing the messages to GCP
PubSub. That can generate a bit of extra work (as multiple processes
might realize it’s time to refresh the JWT and do so), but that
shouldn’t be much. In return, we avoid any possibility of not having
a fresh JWT when pushing messages.
Fixes https://emqx.atlassian.net/browse/EMQX-9905
Since calling `telemetry` is costly in a hot path, we instead collect
metrics inside the buffer workers state and periodically flush them,
rather than immediately as events happen.
Fixes https://emqx.atlassian.net/browse/EMQX-9536
Note: since GCP PubSub is not shared by any authn/authz backend,
there's no need to separate its connector into another app.