[rabbitmq] Emit head_message_timestamp metric when available

[philomory]

Feature Request

The RabbitMQ plugin should support emitting the head_message_timestamp metrics provided for queues by the API, if/when it is available from the broker.

I am preparing a PR to add this functionality; part of the contribution guidelines for making a PR suggests opening an issue first, so, I am doing so here.

Proposal:

The rabbitmq_queue metric should include a head_message_timestamp field (only populated if the metric is actually available).

Optionally, a head_message_age_seconds field could also be emitted, calculated as timestamp (in seconds) - head_message_timestamp. Practically speaking, this is almost always what you actually want to measure, though of course, this could be calculated after the fact using e.g. a Starlark processor, or a Task in InfluxDB.

Current behavior:

The head_message_timestamp field is never emitted as a metric.

Desired behavior:

The head_message_timestamp is emitted as a field of rabbitmq_queue when available.

Use case:

Knowing the timestamp/age of the head message in a queue (which is generally the oldest message in that queue) is absolutely invaluable as a measurement of queue consumer performance, since it allows you to answer the question, "how long does it take for messages to move through the queue?". While in principle this sort of metric can be emitted by the consuming application, the fact that it is often already available at the broker provides an easy way to collect a relevant metric from every queue, regardless of who (if anyone!) is consuming that queue.

[powersj]

next step: final review of #11475