Replace proto.Marshal with direct call to Marshal's

[andrewa-stripe]

Summary

Replace proto.Marshal with direct call to Marshal's

Motivation

Saw veneur prevalent in performance profiles

Test plan

# before: proto.Marshal
BenchmarkSerialization/UDP_plain_span_with_metrics-10         	  464810	      2403 ns/op	     176 B/op	       2 allocs/op
BenchmarkSerialization/UDP_plain_span_no_metrics-10           	  541394	      2488 ns/op	      80 B/op	       2 allocs/op
BenchmarkSerialization/UDP_plain_empty_span_with_metrics-10   	  536200	      2497 ns/op	     112 B/op	       2 allocs/op

# span.Marshal
BenchmarkSerialization/UDP_plain_span_with_metrics-10         	  511609	      2526 ns/op	     160 B/op	       1 allocs/op
BenchmarkSerialization/UDP_plain_span_no_metrics-10           	  635616	      1916 ns/op	      64 B/op	       1 allocs/op
BenchmarkSerialization/UDP_plain_empty_span_with_metrics-10   	  572904	      2157 ns/op	      96 B/op	       1 allocs/op

Rollout/monitoring/revert plan

• PRE merge (if accepted): CHANGELOG.md