Amazon SQS: How to Support context.message.age.ms

The [current messaging spec](https://github.com/elastic/apm/blob/master/specs/agents/tracing-instrumentation-messaging.md#context-fields) includes an optional `context.message.age.ms` field.

> optional for message/record receivers only (transactions or spans). A numeric field indicating the message's age in milliseconds. Relevant for transactions and receive spans that receive valid messages. There is no accurate definition as to how this is calculated. If the messaging framework provides a timestamp for the message- agents may use it (subtract the message/record timestamp from the read timestamp). If a timestamp is not available- agents should omit this field or find an alternative and document it in this spec. For example, the sending agent can add a timestamp to the message's metadata to be retrieved by the receiving agent.
Clock skews between agents are ignored, unless the calculated age (receive-timestamp minus send-timestamp) is negative, in which case the agent should report 0 for this field.

This is not a straight forward metric for the Node Agent to produce. Our `receiveMessage: 'poll',` is not linked to an individual span, and it becomes murky what this value should be. (we do have min/max/avg queue delay metrics which measure the same thing).

We should investigate whether it's possible to support `context.message.age.ms` on individual receiving spans.