Refactor Session Recording Proxy Configuration

[chrisknu]

Summary
This PR changes how Mixpanel session recording determines whether to use a proxy when sending data. It adds a semantically clear session_recording_use_proxy parameter to direct the heavy usage/recording payloads to the proxy with a default of false. If true, it ships to the proxy; else default behaviour.

Changes
Added session_recording_use_proxy configuration parameter (default: false)
Defaults to the direct route to Mixpanel (which is the optimal path for most users)

Why this change?
When using the api_host proxy for capturing/forwarding usage/* events, issues were had trying to pass through the relatively large usage/record event/payloads. Instead of incurring the throughput, its simpler/cleaner to just ship usage/record events straight to mixpanel.

[tdumitrescu]

I think the move here would be to allow each endpoint to specify its own api host, the way we currently allow them to specify their own paths with the api_routes config. You might do this by adding an api_hosts config parallel to the api_routes one (and let those take precedence over the api_host config if present).

[chrisknu]

Good idea. I'll take a swag at that in a few

[chrisknu]

@tdumitrescu
I should probably also mention the below config snippet was problematic in a slightly related way. To get the non-proxy/proxy usage/record URL changes to work, I had to preload the mixpanel-recorder snippet (with api_host changes) in my app to supercede the CDN version (no api_host changes)

'recorder_src': 'https://cdn.mxpnl.com/libs/mixpanel-recorder.min.js',

If/when merged, the config URL shouldn't be a problem but I wanted to see if there was an easier way to config/point to a source (CDN or local in-app asset)

[tdumitrescu]

I had to preload the mixpanel-recorder snippet (with api_host changes) in my app to supercede the CDN version

I'm not sure what that means. The way to switch the location of the mixpanel-recorder bundle for development is to use that config option, e.g.:

mixpanel.init('my token', {recorder_src: 'https://url.of.local.dev.sdk/build/mixpanel-recorder.js'});

[chrisknu]

@tdumitrescu Refactored out the single use config override; added this._mixpanel._getApiHost(<route/type>)