Enhancement: Replace OpenCensus with OpenTelemetry

OpenTelemetry](https://opentelemetry.io/docs/concepts/what-is-opentelemetry/) is an [open standard](https://github.com/open-telemetry/opentelemetry-specification) a sandbox CNCF project and it was formed through a merger of the OpenTracing and OpenCensus.

> OpenCensus and OpenTracing have merged to form OpenTelemetry, which serves as the next major version of OpenCensus and OpenTracing. OpenTelemetry will offer backwards compatibility with existing OpenCensus integrations, and we will continue to make security patches to existing OpenCensus libraries for two years.

There is a lot of outdated documentation as a result of this merger, and we will be better off adopting the latest standard and libraries.

https://github.com/cs3org/reva/pull/1984