Native OTLP intake

[axw]

Add native OpenTelemetry Protocol (OTLP) intake support to APM Server.

We will piggyback on the work done for #3984 to serve on the same port as used for receiving data from Elastic APM agents. Initially we will support OTLP/gRPC, later we can consider extending this to OTLP/HTTP.

The APM Server must be capable of receiving at least traces and metrics over OTLP, as we do with the elastic opentelemetry-collector exporter and we should also consider extending this to receiving logs.

[axw]

I think we're going to have to vendor go.opentelemetry.io/collector in the apm-server tree, renaming the packages (e.g. apm-server/internal/otel_collector/...). The gRPC services are all hidden in internal packages (go.opentelemetry.io/collector/internal/data/...).

[axw]

Opened open-telemetry/opentelemetry-collector#2395 to propose adding a few simple functions to opentelemetry-collector to keep things simple.