Support capturing user-defined metrics

[hmdhk]

trafficstars

Currently there is no way to capture user-defined metrics (key/values) on their own (they need to be added to transactions). Furthermore some metrics do not fit into any single transaction, for example the average XHR response time over the life-time of the page.

Related issues:

• https://github.com/elastic/apm-agent-js-base/issues/33
• https://github.com/elastic/apm-agent-js-base/issues/169
• https://github.com/elastic/apm-dev/issues/264

Further discussion: https://discuss.elastic.co/t/rum-simplified-sending-of-custom-metrics/170470/5

[devcorpio]

For INP we might want to revisit this since it doesn't seem to fit with any of the transaction types we have.

Just thinking out loud 😄

cc: @kyungeunni @vigneshshanmugam