Real-time reporting of metrics using OpenTelemetry

[h-arlt]

In course of #514, support for OpenTelemetry was added to XLT in order to log errors and events in real-time.

This issue is about to extend OpenTelemetry support in XLT for real-time reporting of metrics (parity with metrics reporting using Graphite). On the long run, support for Graphite may be dropped in favor of OpenTelemetry.