Metrics: workaround for prometheus client issue with conflicting sets of labels

[eledhwen]

Introduce a workaround for the known issue with metrics having different sets of labels (prometheus/client_java#696) in which case the micrometer prometheus exporter may end up not showing some metrics (micrometer-metrics/micrometer#4091)

The idea is to do something not unlike what they did over at temporal.io (temporalio/sdk-java#230) where they just add all labels with default values ; in our case that would mean having some mechanism for keeping track of possible label keys for each metric, and set an empty string if the label wasn't explicitly mentioned in the context.