DocumentedMeters + tags for reactor-core-micrometer

[simonbasle]

• add observability doc generation
• Replace constants with PublisherMeters DocumentedMeter
• polish + rewrite the generated meter docs for integration

[simonbasle]

this is an extraction of the resulting docs, as of 1e648c4: Extraction generated metrics docs.pdf

[simonbasle]

tentative commit message:

This commit uses DocumentedMeter API to replace magic String constants
and make the meters and tags in Reactor-Core-Micrometer documented and
public.

The asciidoc generation now takes advantage of this to generate a
section in the reference guide, through the provided Micrometer tool.

Each of the 3 features (meter listener, observation listener and timed
Scheduler wrapper) get their own DocumenterMeter and one associated
doc generation task. We use a Copy task trick on the three resulting
asciidoc files to remove some of the hardcode boilerplate text for a
better inclusion in the reference guide.

Relates to #3108.

[simonbasle]

I noticed in the generated pdf that the javadoc inline html <p> is present in the output text, not treated as markup.

this is an issue with the docs generator, which doesn't do much sanitization/conversion. I have a few ideas for improvements, cc @marcingrzejszczak

[oscar-picnic]

Hi @simonbasle I noticed in this PR that for the MicrometerMeterListener metric %s.flow.duration you have documented it as a distribution summary whereas the actual meter records a timer.

Does it make sense to you to change the documentation to a timer or am I missing something?

See: documentation code