Add a way to enable specific instrumentations rather than rely on disabling

Is your feature request related to a problem?
We have a monorepo with multiple executables sharing a codebase. This means we have many of the opentelemetry instrumentations installed. However we don't necessarily want all instrumentations on all executables. If sticking with opentelemetry-instrumentation, we would need to add to the OTEL_PYTHON_DISABLED_INSTRUMENTATIONS variable every time someone adds a new instrumentation library. This is tedious and error-prone: we can accidentally end up with an instrumentation enabled on some service that we don't want or need.

Describe the solution you'd like
Rather than have to keep adding to the list of OTEL_PYTHON_DISABLED_INSTRUMENTATIONS when running those executables, ideally we can just make the configuration opt-in, and not depend on the ambiently-available instrumentation libraries never changing unexpectedly.

My imagines solution is to add a OTEL_PYTHON_ENABLED_INSTRUMENTATIONS env var with the following semantics:

• the new env var lists instrumentations that are considered; if the env var is not set, all installed instrumentations are considered so that this doesn't affect the current experience for anyone not setting it
• the existing OTEL_PYTHON_DISABLED_INSTRUMENTATIONS prevents the listed instrumentations from being enabled
• it is an error to have a instrumentation in OTEL_PYTHON_ENABLED_INSTRUMENTATIONS that is not installed

Describe alternatives you've considered
I've already added a OURCOMPANY_ENABLED_INSTRUMENTATIONS env var that we use to do this. Ideally this could be pushed up into the library.

Additional context
Add any other context about the feature request here.

Start by tracing how the existing OTEL_PYTHON_DISABLED_INSTRUMENTATIONS setting is read and how installed instrumentations are selected. Implement the requested enabled-list, fallback, exclusion, and missing-installation error semantics, then verify the behavior with the project's instrumentation configuration tests.