Implement hooking of custom telemetry exporters/collectors

[JanProvaznik]

Summary

Enabling hooking of custom telemetry exporting/collection would be useful for customers who want simple observability of builds with OpenTelemetry.
In core hooking OpenTelemetry can be achieved with a Startup Hook https://github.com/baronfel/otel-startup-hook/, in Framework it has to use some workarounds.

Background and Motivation

#11013

• it is one of the goals

Precondition for the utility of this feature is to have data points instrumented with System.Diagnostics Activities. #10946

Proposed Feature

https://github.com/JanProvaznik/otel-startup-hook/tree/doublehook

Productize this demo (probably as a repo with instructions how to use in MSBuild).

Alternative Designs

It could be implemented as part of Microsoft.Build.Framework but that would increase our reliance on OpenTelemetry packages.