Adds a keys method to the NATS message header

[autodidaddict]

On the surface this is just a convenience function, however, by adding the Keys method with this signature, we make the msg.Header field a suitable target for injection of open telemetry span context. In other words, after this change, the Header field will satisfy the TextMapCarrier interface:

type TextMapCarrier interface {

	// Get returns the value associated with the passed key.
	Get(key) string
	// Set stores the key-value pair.
	Set(key string, value string)
	// Keys lists the keys stored in this carrier.
	Keys() []string
}

This means we'll be able to use NATS message headers to automatically convey OpenTelemetry span context via the TraceContext's Inject and Extract functions:

tctx.Inject(ctx, msg.Header)
...
rehydratedContext := tctx.Extract(ctx, msg.Header)

[piotrpio]

LGTM!

[wallyqs]

I think you can do this instead?

tctx.Inject(ctx, otel.HeaderCarrier(msg.Header))
rehydratedContext := tctx.Extract(ctx, otel.HeaderCarrier(msg.Header))

[wallyqs]

This is a small example of how to adapt msg.Header to the otel interface:

package main

import (
	"fmt"
	"net/http"
	"github.com/nats-io/nats.go"
	otel "go.opentelemetry.io/otel/propagation"
)

func main() {
	hdr := http.Header{}
	hdr.Set("foo", "1")
	hdr.Set("bar", "2")
	hdr.Set("quux", "3")
	msg := &nats.Msg{}
	msg.Header = nats.Header(hdr)
	nhdr := otel.HeaderCarrier(msg.Header)
	fmt.Println(nhdr.Keys())
	// => [Foo Bar Quux]
}