Add UDS SOCK_STREAM support to the DogStatsD client

[ddrthall]

What does this PR do?

Includes full support for the unix://, unixstream://, and unixgram:// socket_path prefixes utilized by DD_DOGSTATSD_URL in preparation to support that feature.

Autodetects SOCK_DGRAM vs SOCK_STREAM for users currently providing a raw socket path.

Description of the Change

Alternate Designs

Possible Drawbacks

Verification Process

Set up a datadog agent with both a datagram and a stream uds socket path.
Ensure that a client initialized with socket_path={socket_path} and socket_path=unix://{socket_path} can successfully transmit metrics/events/services to both types of sockets when socket_path is an absolute path and when socket_path is a relative path.
Similarly ensure that socket_path=unixstream://{socket_path} can send to the stream socket and socket_path=unixgram://{socket_path} can send to the datagram socket. Confirm for both prefixes that sending to the wrong socket_path produces an appropriate error.
Ensure for all configurations that invalid socket_paths are error handled in a sane manner.

This process should be repeated for the telemetry flow, where telemetry_socket_path is set rather than socket_path.

Additional Notes

Documentation pushing users to utilize unix://{socket_path} over {socket_path} will be introduced on the inclusion of DD_DOGSTATSD_URL support via https://datadoghq.atlassian.net/browse/AMLII-2173

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.

[vickenty]

What do you think about doing this instead?

Suggested change

	try:
	# This will fail in python2.7
	sk_name = socket_kind.name
	except AttributeError:
	sk_name = socket_kind
	sk_name = { socket.SOCK_STREAM: "stream", socket.SOCK_DGRAM: "datagram" }[socket_kind]

[ddrthall]

Sold