apmotel: follow APM OTel spec and prefer delta temporality

[carsonip]

Make apmotel follow apm otel spec: https://github.com/elastic/apm/blob/main/specs/agents/metrics-otel.md#aggregation-temporality

• Prefer delta temporality on certain instruments
• Filter out zero values on delta temporality

Closes #1436

[axw]

Thanks! This brings us closer to the spec https://github.com/elastic/apm/blob/main/specs/agents/metrics-otel.md#aggregation-temporality

There's also this:

For all instrument types with delta temporality, agents MUST filter out zero values before exporting. E.g. if a counter does not change since the last export, it must not be exported.

If you have time, could you take a stab at that too? Otherwise could you please open an issue?

[carsonip]

For all instrument types with delta temporality, agents MUST filter out zero values before exporting. E.g. if a counter does not change since the last export, it must not be exported.

If you have time, could you take a stab at that too? Otherwise could you please open an issue?

Taken a stab at it, see if it makes sense to you.

[axw]

LGTM with the panic removed. Thanks!

[axw]

LGTM with the panic removed. Thanks!