Create configuration property for common low cardinality KeyValues in Observations

[bclozel]

Currently, Spring Boot automatically registers Metrics Tags for the management.metrics.tags.* configuration properties.
Such values are applied to all outgoing metrics, thanks to a MeterFilter.

We should consider deprecating this property and replacing it with a new management.observations.keyvalues.*. This would apply properties as low cardinality KeyValues for all outgoing Observations (so, not just Metrics). This can be applied using a ObservationFilter bean, in a similar fashion.

[mhalbritter]

I've deprecated management.metrics.tags.* and added a new property management.observations.key-values.*.