Add Unix Domain Socket support to RequestsWrapper

[florimondmanca]

What does this PR do?

Add support for performing HTTP requests to unix://... URLs to RequestsWrapper.

Example:

url = "unix:///var/run/docker.sock"
response = self.http.get(url)

Note: Contrary to what I thought initially, in the end we don't need the user to take any action re: persist_connections. We automatically use the shared session in this case, since we need to mount the adapter on a session and this is not supported by just using the requests top-level API as we do in the default non-persisted case. There's work planned to pay off this bit of tech debt so we build a session in the same way in both cases (more context in #7579 and #7582).

Motivation

Required for the upcoming IoT Edge integration (#7465), and currently marked as TODO in our developer docs.

Additional Notes

QA notes

Note: please QA this on Windows too. If you don't have a Windows machine, either use a Cloud VM (Azure) or kindly pass the card along to someone who has when you're done. 🙏

There's a bug in Docker that prevents sharing Unix sockets between the host and a container (see docker/for-mac#483). This means doing integration/e2e tests on macOS is not possible (only possible on Linux or Windows). If you want to save yourself from spinning up a Linux/Windows VM, here's on way to test this feature using the macOS host Agent…

First, install the macOS RC build on your machine.

Then, run this script to start a local UDS server (Python 3 only):

# Install dependencies: $ pip install uvicorn starlette
import uvicorn
from starlette.responses import PlainTextResponse

app = PlainTextResponse("Hello, World!")

if __name__ == "__main__":
    uvicorn.run(app, uds="/tmp/test.sock")

Verify the server works:

$ curl --unix-socket /tmp/test.sock http://localhost
Hello, World!

Add a custom check that performs an HTTP request to the UDS endpoint:

# ~/.datadog-agent/checks.d/hello.py
from datadog_checks.base import AgentCheck

__version__ = "1.0.0"


class HelloCheck(AgentCheck):
    def check(self, instance):
        r = self.http.get('unix:///tmp/test.sock')
        assert r.text == "Hello, World!"
        self.service_check('hello.can_connect', self.OK, message=r.text)

# ~/.datadog-agent/conf.d/hello.yaml
instances: [{}]

Run a check and make sure it succeeds:

$ datadog-agent check hello
=== Service Checks ===
[
  {
    "check": "my_http.can_connect",
    "host_name": "UPPERlower.com",
    "timestamp": 1600275835,
    "status": 0,
    "message": "",
    "tags": []
  }
]
...

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• 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 changelog/ and integration/ labels attached

[mgarabed]

LGTM!