bug: Fix metric message text for source of message

[jrconlin]

Closes SYNC-4193

[pjenvey]

The way I'm understanding this metric it seems to have the correct name: it's identical to the other message_data metric emitted by incr_success_metrics. They're both called when a message has been successfully delivered, either via webpush or an external bridge.

If anything maybe we should be calling incr_success_metrics here but we reserve its notification.bridge.sent metric solely for external bridges (not webpush). Its third metric notification.total_request_time we emit elsewhere in this module (for some reason, only after trigger_notification_check. I think we forgot to add the same metric for after a successful send_notification call).

I think this should probably be calling a new common function that solely emits the message_data metric that incr_success_metrics would also call? Then both versions would include the same tags (they currently don't).

[jrconlin]

Huh. Good call on having a common function that emits the set of metrics. I'll dig into that.

Mostly, this was just me trying to fix up some of the weird discrepancies around metric calls so centralizing them a bit will help.