Add more details to metrics concept page

[svrnm]

Based on the feedback from #2877, I tried to improve some of the metrics concept page:

• I gave it a structure that is closer to the one we have for traces already (i.e. calling out components like MeterProvider, Meter, etc.)
• Added a metrics support table that (a) provides a link to the languages, and (b) calls out metric stability across them, such that an end user is not going to be surprised if for a specific language they can not find metric instructions

If the latter proves valuable for metrics I can create the same for traces (and later for logs)

[cartermp]

This is a big improvement over the current...sparse...docs.