OpenTelemetry metrics exporter

[dmathieu]

This PR is setting up an OpenTelemetry metrics exporter, based on the specification in elastic/apm#742.

[ghost]

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• Start Time: 2023-04-03T08:42:52.678+0000

• Duration: 3 min 18 sec

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[dmathieu]

Re: CI test failures on Go 1.15, 1.16 and 1.17.
Supporting Go versions down to 1.15 with OpenTelemetry is going to be very tricky.

Otel follows the Golang release policy:

Each major Go release is supported until there are two newer major releases.

Which means it only support 1.19 and 1.20, and has therefore started using generics, making it impossible to use this version with any Go < 1.18.
Going to a lower version of the SDK will not be possible either, as the metrics API wasn't finalized back then.

[axw]

Very nice! Just a couple of comments.

[axw]

@dmathieu great work, thank you. We just need to update the module major version suffixes, otherwise this is good to merge.

We can update the code later based on the outcome of the spec discussion about default bounds.

[axw]

LGTM!