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 436a81d as part of the PR with the implementation of metrics #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.