Support for meters with same names but different tag keys in prometheus registry

[angry-cellophane]

I have an idea how to resolve this issue #877
and raising this PR to discuss it.
The PR has a basic implementation + some simple tests to verify it works and makes sense in general.
I wanted to make sure that the solution makes sense first before going the full path and spending hours or days on making it bullet-proof and efficient. So any input is welcome.

So what this fix does:

1. Current version of micrometer-prometheus creates a collector for each pair of <metric name, list of tag keys>. Collector stores key values dynamically in a map.
2. Prometheus doesn't allow to register two collectors with the same metric name, but it doesn't limit the collector to one particular set of label names (or tag keys in micrometer). The main job of the collector is to return metric samples, which is a list of <tag key, tag value, value> basically. Prometheus supports metrics samples with the same metric name but different tag keys.
3. So if the MicrometerCollector is changed to store tag keys dynamically like it does for tag values now, it can return tag keys and values in samples without any issue and prometheus will be able to parse it correctly.

What I did:

1. Added a new holder for tags to keep a list of keys and values separately for convenience.
2. Changed the map in MicrometerCollector to store the holders instead of tag values.
3. Changed some other dependent methods in MicrometerCollector to get tag keys from the holders.

[nhartner]

It would be great to see this merged. Is there any reason it hasn't been? Not sure who to tag on this but how about it @jonatan-ivanov?

[crolopez]

Any updates? The solution seems simple and would provide great value.

[jonatan-ivanov]

Please see this issue for more details: prometheus/client_java#696
TL;DR: it possible but strongly discouraged.

[zorba128]

Please take a look at prometheus/client_java#901
While I might agree that having two metrics with different sets of labels is somehow problematic, I think this all is deeper problem that its disallowed to have same metrics published by two different collectors (same set of labels, just different label values).
And this is core problem that makes new api really hard to use anywhere but on simple code as on code snippets...