feat: gradually add metrics capabilities to Instrumentation::Base

[zvkemp]

Proposed implementation for adding optional metrics capabilities to existing instrumentation libraries.

See #1377, #1314, or zvkemp#1 (example of observables) for example implementations. (<- note that these PRs are based on top of this branch, so you may see some duplication between here and there)

Proposed api:

• Libraries with metrics must add a boolean option called :metrics.
• Each instrument is configured at compile/boot time — this prevents library authors from creating instruments which are identically named, but have different attributes, callbacks, or advisory parameters, which makes registering them quite difficult. The intention here is that they are declared once in the instrumentation, then subsequent lookups in the instrumentation instance will spawn the instrument (if it has not been spawned already).
• All of the logic in base/metrics works in the absence of metrics-sdk or metrics-api, so library authors can add metrics to libraries without requiring users to use the metrics system at all. This means that any of the instrument-lookup instance methods may return nil (so this will need to be handled, e.g. via short-circuiting https://github.com/open-telemetry/opentelemetry-ruby-contrib/pull/1377/files#diff-6732dfa68439092c8ae9c61ca4dfd067a76e48f9cb6e37768b12aedb5cfb7f50R20)

TODO:

• requires test: test helpers for metrics opentelemetry-ruby#1804
• FIXME should instruments not declared in advance return a valid instrument anyway?
• ensure instrument opts are forwarded correctly, or validate them when they are declared
• dependency load order is very important here; if the Metrics API isn't loaded before Instrumentation::Base, the conditional definitions are not run. We either need to hook into Kernel.require and detect when Metrics is loaded (probably too perilous to really consider, as any instrumentations with conditional metrics will need to be re-evaluated at this time), or eagerly attempt to require the metrics-api gem when Instrumentation::Base is loaded.