[v4] improve metadata documentation (#216)

* add usage docs for context types and metadata, improve comments

* changes after review

metadata/helpers.go

@@ -0,0 +1,7 @@
+package metadata
+
+func copyOf(v []string) []string {
+	vals := make([]string, len(v))
+	copy(vals, v)
+	return vals
+}