FastApi / Starlette setup without client argument not working

[gabriel-f-santos]

Describe the bug:

The ElasticAPM middleware for Starlette appears to be missing a default value for the client parameter, resulting in a TypeError when using the middleware without explicitly providing a client.

To Reproduce

1. Add the ElasticAPM middleware without explicitly providing a client.
2. Observe the TypeError mentioned above.

Environment (please complete the following information)

• OS: Linux
• Python version: 3.11
• Framework and version [e.g. Django 2.1]: fastapi 0.104.1
• APM Server version: ---
• Agent version: 6.20.0

Additional context

Add any other context about the problem here.

Error:

The ElasticAPM middleware should allow for the client parameter to be optional, as indicated in the documentation, and should default to None if not provided.
From docs: https://www.elastic.co/guide/en/apm/agent/python/current/starlette-support.html

from starlette.applications import Starlette
from elasticapm.contrib.starlette import ElasticAPM

app = Starlette()
app.add_middleware(ElasticAPM)

Result:

.venv/lib/python3.11/site-packages/fastapi/applications.py", line 1015, in build_middleware_stack
   app = cls(app=app, **options)
         ^^^^^^^^^^^^^^^^^^^^^^^
TypeError: ElasticAPM.__init__() missing 1 required positional argument: 'client'

in the doc string shows that client its optional in type, but doesnt have None as default value, so passing:

app.add_middleware(ElasticAPM, client=None)

it seems to work.

Proposed Solution

Change the ElasticAPM constructor signature to include Optional[Client] = None for the client parameter, making it optional with a default value of None in https://github.com/elastic/apm-agent-python/blob/4f5661277becc1034ee588bae4b018a4b22cc02b/elasticapm/contrib/starlette/__init__.py#L108C41-L108C41.

def __init__(self, app: ASGIApp, client: Optional[Client] = None, **kwargs) -> None:
    """
    Args:
        app (ASGIApp): Starlette app
        client (Optional[Client]): ElasticAPM Client
    """
    ```

[basepi]

Good find! @gabriel-f-santos, we would love to see a PR if you're willing. You've done all the legwork, so you deserve the credit! But if you don't have time, I can open one.

[gabriel-f-santos]

Good find! @gabriel-f-santos, we would love to see a PR if you're willing. You've done all the legwork, so you deserve the credit! But if you don't have time, I can open one.

Thanks for the quick response! I appreciate the offer, and I'm more than happy to open a pull request to address this issue. I'll work on the changes and submit the PR as soon as possible.