Refactor metrics layer implementation

[adam-cattermole]

Changes

Tried to slightly improve the metrics layer implementation:

• Replaced some unnecessary loops with contains
• Less to_string calls and passing ref
• Removed clone on group_times and instead only clone the keys
• The metrics layer is now disabled for in memory and only runs for the other storage options (as there are no datastore calls and therefore no metrics for in mem)
  • The fmt_layer and telemetry_layer have been moved to independent functions for type inference
• Fixed a bug in setting the level for the telemetry_layer - there were no metrics when the endpoint was provided so it is now set independently on the telemetry_layer (in the new fn)
• Finally changed the datastore spans to be info level apart from for disk storage where they are debug

[alexsnaps]

Some minor remarks... Also wondering, are all the &str 'static? Cause... 🤔
When you bench'ed this, could you see the clones at all in the flamegraphs?