Implement propagators

[bshi]

Presently, trace context via HTTP header X-Cloud-Trace-Context (or the equivalent gRPC header) doesn't appear to be extracted from inbound requests.

OpenCensus equivalent: https://pkg.go.dev/go.opencensus.io/exporter/stackdriver/propagation

https://pkg.go.dev/go.opentelemetry.io/otel/propagators

[glassonion1]

I also agree.
However, I thought that it should be implemented in open-telemetry-go-contrib instead of exporter.

I tried implementing http propagator in my library below.
https://github.com/glassonion1/logz/blob/main/propagation/http_format.go

[punya]

Related to #116.

[punya]

Related to census-ecosystem/opencensus-go-exporter-stackdriver#254 -- we should make sure we comply with the v2 spec.

[amammay]

just so i can have some ease of re-use in some of my own projects i went ahead and created a package based off @glassonion1 implementation with just some slight tweaks to work with open telemetry v0.20.0

https://github.com/amammay/propagationgcp