[otel] automatically propagate span context in `Deno.serve` and `fetch`

[lucacasonato]

By default should be set to OTEL_PROPAGATORS=tracecontext,baggage. Users should be able to override it.

We should support the following propagators initially:

• tracecontext
• baggage

The propagator should also be available via @opentelemetry/api's propagators.

https://opentelemetry.io/docs/languages/sdk-configuration/general/#otel_propagators

[danopia]

I just took a peek at deno's unstable-otel to replace my userland serve/fetch instrumentation, this is so far the first blocker I ran into. Showed up once I sent an HTTP request to Deno thru an otel-enabled api gateway. I'm looking forward to tracecontext propagation landing 🚀