Feat: track number of active requests

Before submitting

- [ ] Was this discussed/agreed via a Github issue? (no need for typos and docs improvements) - [ ] Did you read the [contributor guideline](https://github.com/Lightning-AI/pytorch-lightning/blob/main/.github/CONTRIBUTING.md), Pull Request section? - [ ] Did you make sure to update the docs? - [ ] Did you write any new necessary tests?

What does this PR do?

Track number of active requests
Add hooks on_request and on_response to be triggered on receiving a request and after processing the request resppectively.
By default, tracking active requests is off. Set track_request=True to enable it.

Addresses - https://github.com/Lightning-AI/LitServe/pull/316#issuecomment-2394987019

import litserve as ls
from litserve import LitAPI

class RequestTracker(ls.Callback):
    def on_request(self, active_requests: int, **kwargs):
        # do something with active_requests
        print(f"Active requests: {active_requests}", flush=True)

class SimpleLitAPI(ls.LitAPI):
    def setup(self, device):
        self.model = lambda x: x ** 2

    def decode_request(self, request):
        return request["input"]

    def predict(self, x):
        return self.model(x)

    def encode_response(self, output):
        return {"output": output}

if __name__ == '__main__':
    lit_api = SimpleLitAPI()
    server = ls.LitServer(lit_api, callbacks=RequestTracker(), track_requests=True)
    server.run()

PR review

Anyone in the community is free to review the PR once the tests have passed. If we didn't discuss your PR in GitHub issues there's a high chance it will not be merged.

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Lightning-AI / LitServe