Provide urllib3 implementation of HTTPClient.

[purple4reina]

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What does this PR do?

Adds a third implementation for HTTPClient using urllib3.

Description of the Change

The datadog-lambda-python package depends on datadogpy for submitting metrics in certain cases. The package is used in AWS Lambda environments where resources are very constrained. There is a hard limit of 250MB for all code plus dependencies. Customers are often hitting this limit and so keeping our package size down is important.

One of the largest dependencies of datadog-lambda-python is the requests package and it's dependencies. Removing this dependency would reduce the size of our installed package by roughly 8.5%.

This PR offers a new solution to this problem. We know that urllib3 is installed by default on all python lambda runtime containers, and thus does not count toward the 250MB limit. Switching from requests to urllib3 therefore would allow us to benefit from the 8.5% size reduction.

Alternate Designs

Possible Drawbacks

Verification Process

I created a testing python lambda layer using these changes and then removing requests and dependencies. It is deployed to the serverless sandbox as arn:aws:lambda:sa-east-1:425362996713:layer:Python39-REY:40.

I then created a handler that submits a metric using the datadogpy package. It then returns a list of installed packages and which http client class was used.

import json
import time

from datadog.api.http_client import resolve_http_client
from datadog_lambda.metric import lambda_metric

def get_version(module_name, *version_path):
    if not version_path:
        version_path = ['__version__']
    try:
        module = __import__(module_name)
        for attr in version_path:
            module = getattr(module, attr)
        return module
    except ImportError:
        return None

body = json.dumps(dict(
    requests_version=get_version('requests'),
    protobuf_version=get_version('google.protobuf', 'protobuf', '__version__'),
    urllib3_version=get_version('urllib3'),
    certifi_version=get_version('certifi'),
    idna_version=get_version('idna'),
    chardet_version=get_version('chardet'),
    charset_normalizer_version=get_version('charset_normalizer'),
    http_client_class=resolve_http_client().__name__,
))

def handler(event, context):
    lambda_metric('rey.kittens', 1, timestamp=time.time() - 60)
    return {'statusCode': 200, 'body': body}

The response shows that requests and dependencies -- certifi, idna, chardet, and charset-normalizer -- are no longer installed. Despite removing urllib3 from our layer, we can see that it is still found in the lambda runtime. Lastly, we see that the newly added Urllib3Client is used to submit metrics.

{
  "requests_version": null,
  "protobuf_version": "5.29.2",
  "urllib3_version": "1.26.19",
  "certifi_version": null,
  "idna_version": null,
  "chardet_version": null,
  "charset_normalizer_version": null,
  "http_client_class": "Urllib3Client"
}

The metric explorer shows the custom metric rey.kittens has been ingested.

Additional Notes

This change will have many additional positive impacts as well. We see a 12% reduction in cold start time for example.

Release Notes

Review checklist (to be filled by reviewers)

• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
• PR should not have do-not-merge/ label attached.
• If Applicable, issue must have kind/ and severity/ labels attached at least.