Support for OTEL_PROPAGATORS environment variable

[krak3n]

Currently the propagators are hard coded to W3C TraceContext and Baggage within the tracing module here.

This makes it difficult to integrate Caddy into existing systems that do not use W3C TraceContext or Baggage for propagating traces.

This PR adds support to the caddyhttp/tracing module to support the OpenTelemetry OTEL_PROPAGATORS environment variable which allows users to configure the propagators used to extract and inject trace data from incoming and outgoing requests.

For example, users will now be able to define their propagators like this:

export OTEL_PROPAGATORS=b3,b3multi,baggage

As defined here on the OpenTelemetry SDK configuration document.

This is achieved by using the go.opentelemetry.io/contrib/propagators/autoprop propagator which also uses the W3C TraceContext and Baggage as the default if no propagators are set on the OTEL_PROPAGATORS environment variable, see here

[CLAassistant]

All committers have signed the CLA.

[mholt]

Thanks for the PR. I will need to check on something before we merge this, as the otel dependency is a bit tricky for some of our users.

[starkers]

Thanks for the PR. I will need to check on something before we merge this, as the otel dependency is a bit tricky for some of our users.

Any progress on this @mholt ? support for b3 (or others) is quite important

[mholt]

We can merge this, but the lint errors need fixing.

Edit: Nevermind -- just looked and for some reason the linter has started complaining about code unrelated to this change. 🤔

I'll look into this...

[mholt]

I don't think those linter errors make any sense. That's got to be a bug. The code shouldn't even be compiling if some of those are true.

I'll merge this and cross fingers. Thanks @krak3n !

[starkers]

I'll be testing this today, maybe prod next week ;-)

[krak3n]

Awesome thank you @mholt 🎉