gradually add metrics capabilities to Instrumentation::Base

[zvkemp]

[description WIP; still amending commits]

TODO:

• FIXME: decide what (if anything) to return if Metrics is undefined
• 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.