Fix B3 propagation headers injection

[pellared]

What

Add leading zeros to the B3 propagation x-b3-traceid and x-b3-spanid headers so that they are always 16 characters long as required by the specification.

Benchmarks

func BenchmarkToHex(b *testing.B) {
	for i := 0; i < b.N; i++ {
		toHex(rand.Uint64())
	}
}

Current implementation

func toHex(value uint64) string {
	b := make([]byte, 8)
	binary.BigEndian.PutUint64(b, value)
	return hex.EncodeToString(b)
}

Running tool: /usr/local/go/bin/go test -benchmem -run=^$ -bench ^(BenchmarkToHex)$ github.com/signalfx/signalfx-go-tracing/ddtrace/tracer

goos: linux
goarch: amd64
pkg: github.com/signalfx/signalfx-go-tracing/ddtrace/tracer
cpu: Intel(R) Core(TM) i9-10885H CPU @ 2.40GHz
BenchmarkToHex-16    	21657834	        55.59 ns/op	      32 B/op	       2 allocs/op
PASS
ok  	github.com/signalfx/signalfx-go-tracing/ddtrace/tracer	1.268s

Alternative implementation

func toHex(value uint64) string {
	return fmt.Sprintf("%016x", value)
}

Running tool: /usr/local/go/bin/go test -benchmem -run=^$ -bench ^(BenchmarkToHex2)$ github.com/signalfx/signalfx-go-tracing/ddtrace/tracer

goos: linux
goarch: amd64
pkg: github.com/signalfx/signalfx-go-tracing/ddtrace/tracer
cpu: Intel(R) Core(TM) i9-10885H CPU @ 2.40GHz
BenchmarkToHex2-16    	 8361780	       142.4 ns/op	      24 B/op	       2 allocs/op
PASS
ok  	github.com/signalfx/signalfx-go-tracing/ddtrace/tracer	1.343s