Consolidate router success metrics code into a common place

[data-sync-user]

Renamed from original issue “Rename storage source metric”

Currently, we use ...notification.message_data for both the storage source of the message as well as the type of message that we are passing through the bridge. This leads to confusion when reading the metric panel.

We discovered this metric isn’t necessarily misnamed but instead we should consolidate the router success metrics code into a single common place (see https://github.com/mozilla-services/autopush-rs/pull/660#pullrequestreview-2056625094 )

┆Issue is synchronized with this Jira Task

[data-sync-user]

➤ Philip Jenvey commented:

Moving this from https://mozilla-hub.atlassian.net/browse/SYNC-3448 ( https://mozilla-hub.atlassian.net/browse/SYNC-3448|smart-link ) to https://mozilla-hub.atlassian.net/browse/SYNC-3277 ( https://mozilla-hub.atlassian.net/browse/SYNC-3277|smart-link ) as this issue isn’t quite what we thought it was (badly named metric) – it’s more of what I’d say is a refactor.