Add opentelemetry support

[glazychev-art]

Description

This PR adds opentelemetry support.
At the moment, you can use either opentracing(jaeger) or opentelemetry functional. The latter also provides the ability to work with metrics.

How does it work?

The choice of using one of the functional is made through the environment variable.

Opentracing is used by default (selected as more stable).

Opentelemetry initialization is available through opentelemetry package. It creates tracer and meter providers.
Next, spanlogger uses corresponding spans for tracing, and metrics chain element works with metrics from pathSegment.
Traces and metrics are sent to otel-collector. You can add agents if necessary.

                                                      -----> Jaeger (trace)
App + (SDK + otel-SDK) ---> OpenTelemtry Collector ---|
                                                      -----> Prometheus (metrics)

So you also need to run otel-collector container to receive traces and metrics.

How to enable opentelemetry?

1. Enable traces.
2. Set env TELEMETRY = opentelemetry.
3. Call opentelemetry.Init(...).

Signed-off-by: Artem Glazychev artem.glazychev@xored.com
#584

[edwarnicke]

@glazychev-art Is this still blocked? If not, should we remove the blocked tag?

[glazychev-art]

@edwarnicke This is still blocked by #744