PubSubSink use timeouts to set unhealthy sink

[istreeter]

The collector has a sink-health endpoint, which signals when the sink becomes unhealthy (even though webserver itself is completely healthy). Until now, the pubsub sink is marked as unhealthy only when the publisher has completely failed to sink some events. However, we are changing configuration so that by default the publisher retries indefinitely to sink all events, and therefore the sink would never get marked as unhealthy.

So we need an alternative algorithm to mark when the sink is unhealthy. This can be done by having a timeout, so if the event is not sunk within, say, 10 seconds then we mark the sink as unhealthy.