Adding known values for OTEL_PROPAGATORS

[codeboten]

Changes

Adds known propagator values to the environment variables specification.

[carlosalberto]

Overall LGTM - however, I think we need to add a value to specify B3's formats (single and multiple). As the single part is the default, probably we should have something like:

• b3 specifies single header format.
• b3multi (or similar) specifies multiple format.

EDIT: The B3 case may disappear, depending on whether #961 is merged ;)

[carlosalberto]

@open-telemetry/specs-approvers Please review this PR.

[arminru]

@carlosalberto Should we wait for #961 to settle then or go with just b3 for now?

[carlosalberto]

@arminru As there seems to be initial agreement on having a single B3 Propagator, I'd suggest we go with b3 for now (if that changes, we can always add a b3single or b3multi very fast afterwards).