Try out measured for metrics

[clux]

A working POC for conradludgate/measured ripping out tikv/rust-prometheus. It did take me about 3h of cranking to make everything work like before, but it's a one time cost.

PROS:

• great crate docs with a specialised docs chapter with guides/getting started/examples
• nice type safe api around labels, i.e. CounterVec
• best performer on cpu/memory performance - going by upstream benchmarks
• benchmarks against the major metrics crates in benches
• by someone i trust to write good code, and has a chance of being maintained (tikv one is kind of maintenaned)
• Histogram Timers out-of-the-box (could rip out our old ad-hoc ReconcileMeasurer scope thing)
• metric groups allow namespace prefixing groups of metrics ala neon metrics
• supports locking down cardinality with FixedCardinality
• can force it do take defaults everywhere to make DynamicLabels reasonably ergonomic

Note that the fixed cardinality stuff is not something we actually use, nor a main focus atm, as a controller benefits from dynamic labels for the objects it is reconciling and they are generally bound the cluster size anyway. But I can see it being useful for constraining error sizes down the road (the current error_label thing is very lazy and could blow up a bit under bad conditions).

CONS:

• complexity overhead for dealing with interned strings and lasso (dependency is exposed so you have to understand this) - Expose lasso from measured to avoid sibling dependencies conradludgate/measured#1
• complexity dealing with labelgroup derive creating hidden structs with different names (confused me the most when migrating) - Could the LabelGroup derive re-use the user-defined struct? conradludgate/measured#3
• reflection for getting values for unit tests are a little awkward / but on plus side no registries!
• protobuf WIP: conradludgate/measured@3b4a440
• not sure if it is compatible with openmetrics format (only encoder)
• no exemplar support (exists in the official, requires openmetrics) - only loosely referenced in generated protobuf code - Add Exemplar support conradludgate/measured#4
• does not build upon the official client so uncertain if future/potential native histogram would be propagated (but from benchmarks the official client is also not great...)

I believe opemetrics text format with exemplars could be easily retrofitted on top of measured, so maybe that's a way forward.