Document how to expose metrics in a custom exporter

[npepinpe]

Description

With 8.6.0, we added a new functionality where exporter authors can easily add new metrics that will be exposed alongside all the other C8 metrics (e.g. via Prometheus), using the Context#getMeterRegistry().

We should document:

• That this feature exists
• What it is for
• How to use it, via an example

This should be added to the next version and to 8.6.x, and released as soon as possible, as it covers existing functionality.

[npepinpe]

@EuroLew - pre-assigning under the assumption this is not a big thing. Not super urgent, let's try and get it done sometime next week.

[akeller]

Added the 8.6.0, mostly for DevEx review purposes when this is PR'd.

[akeller]

@EuroLew this was will be closed by #4551, right?

[EuroLew]

@EuroLew this was will be closed by #4551, right?

yes