Analytics adapters: attach arbitrary labels to analytics events

[dgirardi]

Type of change

• Feature

Description of change

This allows attaching arbitrary labels to all analytics events, to facilitate distinguishing between experiment groups. For example, setting

pbjs.setConfig({
   analyticsLabels: {
      "some_experiment": "some_group"
   }
})

will cause all event payloads seen by analytics adapters to contain

 {
   // ...
   analyticsLabels: {
     "some_experiment": "some_group"
   }
 }

Additionally, adapters that extend the base AnalyticsAdapter's track method will see the same label data come in as a new labels parameter, e.g. track({eventType, args, labels}). (Unfortunately the nicer name labels is already taken in event payloads, where it sometimes refers to sizeMapping labels).

Other information

Closes #10705

[patmmccann]

Cc @3link