Improve defaulting for propagation

[dashpole]

Related to #1391

Currently, installing an SDK and plugins, such as otelhttp does not install any propagators. I encountered this in my personal experience trying to instrument a web service, and force sampling by sending a sampled trace context. I couldn't figure out why it wasn't working, and it took me a day of debugging to realize I needed to install a propagator. It is difficult to debug when this doesn't work. A user from gitter also ran into this problem, and this issue is based on a discussion there.

However, we can't set a default in the API: https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/context/api-propagators.md#global-propagators
The OpenTelemetry API MUST use no-op propagators unless explicitly configured otherwise.

We could try and set the global propagator in NewTracerProvider, but that would have undesired behavior when using multiple TracerProviders, and might cause confusion if used in conjunction with SetTextMapPropagator(), since the default could clobber the user-set one. We could also do it in InstallNewPipeline for various export pipelines. This is probably better, since it also sets the global TracerProvider.

cc @MrAlias @jmacd @Aneurysm9

[dashpole]

also related: #1265

[MrAlias]

Likely going to be related to this part of the SDK specification

General SDK Configuration

Name	Description	Default	Notes
OTEL_PROPAGATORS	Propagators to be used as a comma separated list	"tracecontext,baggage"	Values MUST be deduplicated in order to register a Propagator only once. Unrecognized values MUST generate a warning and be gracefully ignored.

[Aneurysm9]

Likely going to be related to this part of the SDK specification

General SDK Configuration

Name
Description
Default
Notes

OTEL_PROPAGATORS
Propagators to be used as a comma separated list
"tracecontext,baggage"
Values MUST be deduplicated in order to register a Propagator only once. Unrecognized values MUST generate a warning and be gracefully ignored.

Additional values can be specified in the respective SDK's documentation, in case third party Propagators are supported, such as "xray" or "ottracer".

That sounds like we'd need to provide a registration mechanism for third-party propagators.

[MrAlias]

Can this be closed in favor of recommending https://pkg.go.dev/go.opentelemetry.io/contrib/propagators/autoprop @dashpole ?

[dashpole]

yes, definitely.