metrics: make sure metrics work well with sharding

[p-se]

We need to ensure that metrics are collected against resources reconciled not just by the unsharded Fleet controller, but also by sharded controllers.

~~This could be an opportunity to improve shard label naming, for instance by supporting label-based filtering on the unsharded controller (currently not possible with an empty shard value). For instance, we could use ~ instead of an empty string, or simply add a label such as fleet.cattle.io/empty-shard=true or similar.~~

A new label shard-default with value true for the unsharded controller has been implemented in https://github.com/rancher/fleet/pull/2172/commits/436a81de5f452183e607e3983cffd7f02ee88bd0 as part of the PR with the implementation of metrics https://github.com/rancher/fleet/pull/2172. Metrics still does not work with sharded fleet controllers. This is to be remedied.

This is a follow-up on both #1740 and #1408.

[weyfonk]

Note: this documentation on how Flux had planned to do it may be useful.