Constructor guards

[Lewiscowles1986]

Due to a mis-configuration at work, our agent host was in some cases not being set.

This led to (luckily dev/qa time) discovering a PHP warning is thrown at runtime if the class is constructed with an incorrect value.

Like a lot of 12-factor apps, we get our value from ENV, which in our case resulted in no errors and a booting app. But warnings silently output.

I think the constructor of DogStatsTimer should throw if the host configuration is not a valid hostname or IP address. In our case it was false 😂

I Know people SHOULD NOT have warnings output in QA or production environments. But separate to that issue, I would be interesting in contributing this change if the authors are amenable.

[gh123man]

Sounds reasonable to me.
Feel free to open a PR or if you feel like the contribution is non-trivial we can discuss the approach here.

[Lewiscowles1986]

Okay, so I have a candidate proposal that does not mess with the existing logic, but reorganizes it a bit, gets coverage of new code based upon that; I Hope it not too adventurous, and has some (separated commits) for other things that came up in solving this, but are entirely optional, and can be dropped.

https://github.com/Lewiscowles1986/php-datadogstatsd/tree/feature/injectable-transport

• Proposed code changes (minimal, interface based)
• Test fix (I was authoring on Windows, and it was a pain, I could not run tests. It's 2-liner)
• GitHub actions (until I'd fixed, I actually had a really slow feedback loop)
• Editor config and git attributes
• Proposed Documentation (words are hard, forgive me if it's not perfect).
• Update to support PHP 8.0 & 8.1 thanks to Yoast

Tests

• PHP 5.6 - PHP 8.1
• Linting, Code-Style

The actual guard portion of this, doesn't have to be owned by this repo. In the README.md I added a section with example to bypass the magic, re-using the Ipv4Udp transport that is part of this PR, so that an example of instrumenting the control I was looking for (bypassing the magic in current constructor) is available.

I'm not 💯 sure about the construct with single method interface, but felt that opening and closing the socket separately to the functionality; could make some future testing easier. I Also didn't want to open a long-lived socket (although the classes don't hint at that, the way they are used in one place enforces for-now).

The interfaces are pretty stripped-back to deal with PHP 5.6 minimum. Not sure where DataDog is going with that, but I think users would get a lot of benefit rolling up to only 7.4 & 8+ in a future release.

[gh123man]

Thanks for the comprehensive work. For our record keeping, can you please update the PR description with everything you mentioned above?

Also thanks for including the fixes for php 8 + PHPUnit.

Ill leave the rest of my feedback in the PR itself.