ochttp public traceparent/tracestate marshal/unmarshal

[slinkydeveloper]

NB: Before opening a feature request against this repo, consider whether the feature should/could be implemented in OpenCensus libraries in other languages. If so, please open an issue on opencensus-specs first.

Is your feature request related to a problem? Please describe.
I wish to read and write traceparent/tracestate, encoded using trace context, without passing an http.Request

Describe the solution you'd like
Two new methods should be added to HttpFormat or as public functions:

SpanContextFromHeaders(traceparent string, tracestate string) (sc trace.SpanContext, ok bool)
SpanContextToRequest(sc trace.SpanContext) (traceparent string, tracestate string)

Describe alternatives you've considered
AFAIK no alternatives are available in this library, except copy pasting that code in my own code.

[llhhbc]

This method has already has in package: go.opencensus.io/plugin/ochttp/propagation/b3

HTTPFormat{}

func (f *HTTPFormat) SpanContextFromRequest(req *http.Request) (sc trace.SpanContext, ok bool)

func (f *HTTPFormat) SpanContextToRequest(sc trace.SpanContext, req *http.Request)

[slinkydeveloper]

What I mean is that I want to extract manually the traceparent/tracestate headers and then i want to pass them to a method, while these two methods interacts already with http request