Consistent way of not instrumenting multiple times

[lzchen]

Fixes #439
We use _is_instrumented_by_opentelemetry for all functionality involving indicating whether a component has been instrumented. This is the text that should be used for any context variable/property that is used revolving around determining whether an instrumentation is already instrumented or not.

Another thing to note is the relationship between instrument() and instrument_app/instrument_connection() etc.
instrument_X() should take priority over instrument(), i.e. if uninstrument_app(app) is called after instrument(), app should produce no telemetry.

As well, calling instrument_app(app) after instrument() should not produce double telemetry.

Also a bunch of other consistency-based changes:

1. Made all instrument/uninstrument_X static methods, where X is a component
2. fastapi - added uninstrument_app
3. Changed warning to error on dependency conflict for instrument()

[codeboten]

We use the string _is_instrumented_by_opentelemetry for all functionality involving indicating whether a component has been instrumented.

@lzchen this statement in the PR description was confusing to me, that variable is a boolean, not a string correct?

[lzchen]

@codeboten
Yes it is a boolean. In the description I was outlining that we are using this specific text_is_instrumented_by_opentelemetry (instead of some other text like is_instrumented) for all things related to instrumenting. I've updated my comment.

[codeboten]

Should this be specified in the spec or is this implementation specific details? https://github.com/open-telemetry/oteps/pull/165/files?short_path=fac8c59#diff-fac8c5914f3523b6888b428ec9eea8229238297768d93e2171418f7c45f805ce

[codeboten]

👍 Thanks for addressing my comments