Consider having a map and request header Carrier in SDK

[rakyll]

The propagation package is a highly generic package and it requires users to implement common cases from scratch by implementing the interfaces. Two of these interfaces are implementation with the builtin map type and a carrier that works with a request object.

It's be useful to provide two Carrier implementations out of the box from the SDK or somewhere else:

func NewMapCarrier(m map[string][]byte) propagation.Carrier
func NewHeaderCarrier(h http.Header) propagation.Carrier

[Aneurysm9]

We do provide a HeaderCarrier, but having a carrier implementation for map[string][]byte could be a nice addition.

[tobert]

this would be so nice, I've written this a few times now for various projects and it feels weird to write & test a string map wrapper each time when it's the same every time and a lot of folks doing interesting things with trace propagation need it