feat: add support for OTel context propagation and harmonized spans

[feywind]

This PR adds support for OTel context propagation using the standard W3C headers as Pub/Sub attributes. It also harmonizes the spans we're using with the planned spans in other language libraries.

Fixes #1389

(This is the 4.x version of #1659 - I'll still be backporting to there, but the changes will go here first.)

[feywind]

@naseemkullah The delay has been annoying for sure. The good news is, this is being reviewed in pieces and there will hopefully be a beta release soon.

[feywind]

There's one more of these in the review queue right now:
feywind#5

Some fabulous people on our side have been working to harmonize our span design a bit more with the OTel working group's design in progress for message tracing. :)

[feywind]

@hongalex The sample needs updating, but this should more or less be the finished PR now. Thanks for all the review help.

[feywind]

This is now testable in beta form. I installed in a test project like so:

npm i --save @google-cloud/pubsub@otel-beta

The resulting package.json looks like this:

{
  "dependencies": {
    "@google-cloud/opentelemetry-cloud-trace-exporter": "^2.1.0",
    "@google-cloud/pubsub": "^4.3.3-otel-beta.1",
    "@opentelemetry/sdk-trace-node": "^1.23.0",
    "@opentelemetry/semantic-conventions": "^1.23.0"
  }
}

And then you can take a look at this sample for how to use it from JS, or this sample for how to use it from TS.

We've got more reviewing to do, and merges aren't going to happen for at least a week, but this may at least make it easier to do some testing if you like.

Further updates (for now) will come in as 4.3.3-otel-beta.2, etc, and will update the tag otel-beta on npm.