Add installed_base_by_architecture and weekly_installed_base_by_architecture metrics

[LukeWH]

We'd like to provide additional metrics to allow publishers to see the number of active devices on multiple architectures - this should help them when deciding which architectures they should support.

Additional 'metric_name's on the [POST] /dev/api/snaps/metrics endpoint, could be:
installed_base_by_arch
weekly_installed_base_by_arch

Cheers!

[Przemysław Suliga (suligap)]

`weekly_installed_base_by_architecture` seems to be something that is already being collected, but not yet exposed via the stats service.

[Przemysław Suliga (suligap)]

LukeWH,

We've done the change that should expose the `weekly_installed_base_by_architecture` metric and are working on getting it released.

Do any of you know if there's a reason for the "weekly" already existing while the corresponding daily one is missing? Do we definitely want to add the daily one too?

[Przemysław Suliga (suligap)]

The new metric names are:

- installed_base_by_architecture
- weekly_installed_base_by_architecture