Accept OpenTelemetry trace data via OTLP

[yurishkuro]

This is the most important feature that came up in Jaeger v2 discussions, but it does not actually require v2, can be done standalone.

• Add OTLP receiver to collector Add OTLP receiver to collector #3701
• Update documentation Give more emphasis to OpenTelemetry documentation#578
• Add OTLP receiver to agent - let's wait for user demand
• Support configuration of the receiver (ports) Introduce OTLP receiver configuration flags #3710

[joe-elliott]

Where do we see this going? otlp grpc endpoints on the agent make the most sense to me.

[yurishkuro]

I would mount it both in the agent and in the collector. Some deployments (e.g. serverless) do not support agents.

[ybasket]

Where would the user demand for OLTP on the agent be collected? I'd love to see this feature implemented.