docs: adds note about defaultTags (#519)

Co-authored-by: Crypt Keeper <64215+codefromthecrypt@users.noreply.github.com>

Author: jcchavezs

packages/zipkin/src/tracer/index.js

@@ -39,6 +39,7 @@ class Tracer {
     localEndpoint,
     /* eslint-disable no-console */
     log = console,
+    // defaultTags custom tags to add to all sampled spans. Defaults to empty.
     defaultTags
   }) {
     this.log = log;