feat: Make instrumentator work with just Starlette

[mvanderlee]

What does this do?

This PR removes FastAPI and uses Starlette instead (which FastAPI extends).

Why do we need it?

This means that non-FastAPI projects don't have to install all it's dependencies.

Who is this for?

Non-FastAPI users

Linked issues

Fixes #280

Reviewer notes

--

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.79%. Comparing base (885c80c) to head (a1f989b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #288 +/- ## ========================================== + Coverage 95.68% 95.79% +0.10% ========================================== Files 5 5 Lines 348 357 +9 ========================================== + Hits 333 342 +9 Misses 15 15 ```

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[trallnag]

Hi, thanks for proposing and implementing this

Note that it is not possible to remove any trace of FastAPI from the instrumentator, as the expose() method relies on FastAPI-specific parameters. I could remove it, but I would prefer to keep it backwards compatible.

Nevertheless I managed to remove the dependency on the fastapi package by dynamically checking if the app is Starlette or FastAPI instance and only importing fastapi, if it is a FastAPI app. I added the changes to your PR.

It should be backwards compatible

[mvanderlee]

Nice, awesome work @trallnag!