Add DD_DOGSTATSD_URL support for unix and udp urls

[ddrthall]

Adds support for the unix:// and udp:// variants of DD_DOGSTATSD_URL.

Description of the Change

During dogstatsd creation, utilize the DD_DOGSTATSD_URL environment variable if the provided host and port are their default values and no socket_path has been provided. unix:// and udp:// paths are supported, but \\.\pipe\ paths (windows named pipes) are not yet implemented.

Alternate Designs

Possible Drawbacks

Verification Process

Form a Dogstatsd object with different values of DD_DOGSTATSD_URL. The following cases are important to verify:

• A non-default host, port or socket_path provided to the Dogstatsd constructor should take precedence over DD_DOGSTATS_URL.
• unix:// prefixed DOGSTATD_URLS should result in a socket_path and a None host/port pair
• udp:// prefixed DOGSTATSD_URLS should result in a valid host/port pair, with appropriate type safety around casting the port component from a string to an integer.
• \\.\pipe\ prefixed DOGSTATD_URLS should provide an informative debug message and fallback to other connection identifications.
• An unrecognized prefix for DOGSTATD_URL should fallback to other connection identifications

During fallback, DD_AGENT_HOST and DD_DOGSTATSD_PORT should be utilized if present. Otherwise, udp is selected with DEFAULT_HOST:DEFAULT_PORT.

Additional Notes

This PR is reliant on the functionality in !869, with a comparison available here

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.