Meter documentation

[stickeegreg]

It's hard to figure out from the documentation what a Meter is.
It doesn't appear in the Glossary and the page concepts/signals/metrics page doesn't have any detail.
It would be nice to see something a long the lines of this (though I don't know if this is correct as I'm still not sure I understand it properly).

Meter

A Meter creates metric instruments, capturing measurements about a service at runtime. Meters are created from Meter Providers. Each Meter belongs to an Instrumentation Scope, which it used to group Instruments.

[svrnm]

Hey @stickeegreg thanks for raising this issue.

If you like, raise a PR with your suggested change and we can take a look from there!