[receiver/datadog] Implement '/stats' endpoint

[lopes-felipe]

Description:
Following up on #34772, this PR implements Datadog Agent's /stats endpoint, which is used for sending client stats of the local sampled spans to calculate the trace metrics.

This PR also does a minor refactor in the usage of Datadog's common headers, moving from literal strings to centralized constants.

Link to tracking Issue:

Testing:

Documentation:

[lopes-felipe]

@jpkrohling, it looks like I'm having the same issues here. I have tried your make lint and make gotidy spells, but no luck. Any other magic words here that could please the CI Lords? 🙂

[jpkrohling]

I can run those for you, but can you try running make gotidy at the root of the project? It should take a while, but will run it on modules that consume this receiver, like the otelcontribcol. If you are using go 1.23, make sure the toolchain entry doesn't change (I typically need to revert it manually).

[lopes-felipe]

I can run those for you, but can you try running make gotidy at the root of the project? It should take a while, but will run it on modules that consume this receiver, like the otelcontribcol. If you are using go 1.23, make sure the toolchain entry doesn't change (I typically need to revert it manually).

Thanks for the response. Just ran make gotidy at the root, and made no changes. I'm using go 1.22.4.

[jpkrohling]

Sorry about the merge conflict. Would you be able to address it? Once it's done, I'll merge on green.

[lopes-felipe]

Sorry about the merge conflict. Would you be able to address it? Once it's done, I'll merge on green.

No problem! Thanks for the help :)