Add instrumentation patch metrics

[delner]

As a further extension of #867, this pull request refactors how integration patching works, and adds a metric for whenever integration patching succeeds/fails.

New metrics include:

• datadog.tracer.instrumentation_patched: Whenever a Datadog::Contrib::Patcher#patch succeeds.
  • patcher: Patcher that was applied.
  • target_version: Version of software that was patched (typically a gem version.)
• datadog.tracer.error.instrumentation_patch: Whenever a Datadog::Contrib::Patcher#patch raises an error.
  • patcher: Patcher that was attempted.
  • target_version: Version of software that patch was attempted against (typically a gem version.)
  • error: Class name of error raised.

[delner]

This is now ready for review.

I think this is fine as is, but it might be worthwhile also adding a "gem version" tag to the metric of the library that failed to patch. e.g. If our Rails integration failed to patch Rails 6.1, it would add a target_version:6.1.0 tag to the error metric.

[marcotc]

Any particular reason for handling the case where the Patcher is a class, given all our current Patchers are modules?
Would it be for future proofing?

[delner]

Yeah, technically our integration API doesn't require all patchers to be modules, just that they include Datadog::Contrib::Patcher and behave like one. This means users who have created custom integrations might have implemented the patchers as classes: it's more of a "avoiding possible breaking change" thing than future proofing.

[delner]

Added target_version tag and an instrumentation_patched metric which is emitted whenever an integration's patch succeeds.