Do not de-dot metrics if communicating with APM Server 8.3+

[eyalkoren]

Context: elastic/apm#347 (comment)

If communicating with APM Server 8.3 or newer, we should not dedot metrics by default.
If there's no good reason to allow dedotting through configuration, we may simply ignore the dedot_custom_metrics in such cases.

[axw]

The changes that could enable this exist in Elasticsearch, but we're not yet using them. We'll need to wait for a later release.

[eyalkoren]

Right, I jumped the gun on this...
Do you have an APM Server issue to link to?

[axw]

Not yet -- I'll open one when elastic/apm#347 (comment) is resolved.