Metrics & troubleshooting guide on Elasticsearch backpressure

[carsonip]

As ES indexing moved from libbeat to docappender a long time ago, we no longer get "queue is full" log messages when apm-server internal buffer is full.

• With docappender, we may / may not have metrics available to identify ES backpressure. Confirm whether existing metrics are good enough. Backpressure may come in a form of 429, or increased ES latency.
• Either way, ensure that there is documentation on how to identify and troubleshoot such scenario.