starlette: Replace _meter_provider by meter_provider on _instrument (open-telemetry#3048)

* starlette: Replace `_meter_provider` by `meter_provider` on `_instrument`

* Add PR on the changelog

---------

Co-authored-by: Riccardo Magliocchetti <[email protected]>

Author: Kludex

Diff for: CHANGELOG.md

@@ -32,6 +32,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- `opentelemetry-instrumentation-starlette`: Retrieve `meter_provider` key instead of `_meter_provider` on `_instrument`
+  ([#3048](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3048))
 - `opentelemetry-instrumentation-httpx`: instrument_client is a static method again
   ([#3003](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3003))
 - `opentelemetry-instrumentation-system_metrics`: fix callbacks reading wrong config

Diff for: instrumentation/opentelemetry-instrumentation-starlette/src/opentelemetry/instrumentation/starlette/__init__.py

@@ -265,7 +265,7 @@ def _instrument(self, **kwargs):
         _InstrumentedStarlette._client_response_hook = kwargs.get(
             "client_response_hook"
         )
-        _InstrumentedStarlette._meter_provider = kwargs.get("_meter_provider")
+        _InstrumentedStarlette._meter_provider = kwargs.get("meter_provider")
 
         applications.Starlette = _InstrumentedStarlette