Add/setting source name

[ladanazita]

Millicom asked us to add a custom "segment_source_name" property to our client and serverside Mixpanel integrations. Today, this feature only works for the server-side Mixpanel integration.

The changes introduced will support sending a property called segment_source_name to Mixpanel on a Segment track event when a user has the setting sourceName configured with a value in Segment's UI. This brings the client and server-side Mixpanel integrations to parity with regards to this setting.

Tested locally with golden-compiler
Steps taken:

1. Install tool with npm install -g @segment/golden-compiler
2. Create settings.json and add the following

{
  "Mixpanel":{
    "consolidatedPageCalls":true,
    "crossSubdomainCookie":true,
    "eventIncrements":[],
    "legacySuperProperties":false,
    "people":false,
    "peopleProperties":[],
    "persistence":"cookie",
    "propIncrements":[],
    "secureCookie":false,
    "setAllTraitsByDefault":true,
    "sourceName":"my_source",
    "superProperties":[],
    "token":"<YOUR TOKEN>",
    "trackAllPages":false,
    "trackCategorizedPages":false,
    "trackNamedPages":false}
}

3. Enable Mixpanel in a sample workspace/source
4. Add value to the setting sourceName, in this case I added my_source.
5. Using the relevant writeKey, run: golden-ajs --settings= ./settings.json --slug=mixpanel --write-key=<WRITE_KEY>
6. Open localhost:8080
7. See the local version of AJS loaded with your updates in the source tab
8. In the console, trigger a track call
9. See the request go to Mixpanel in the network tab
10. See the source_name appear in Mixpanel's UI as a property

JIRA ticket: https://segment.atlassian.net/browse/ENT-97
CC ticket: https://segment.atlassian.net/browse/CC-922

Note: The sauce lab keys are removed here because the ENV variables are respected and set via Circle CI's UI, not the circle.yml. In addition to this, we should not put credentials in open sourced repos. Confirmed that these credentials removed are expired and no longer used in production.

[ucarion]

Great PR! Looks good to me.

Let's be sure to make the relevant setting rendered in CDN. :) I'm not sure if we should make it hidden or not, though.

[ladanazita]

Also tested with a page call, which sends in as track to Mixpanel:

Here is the source_name property appearing in Mixpanel after the page call is triggered: