Add public, read-only access to label names, label values of any metric

Hi,

I am currently working on Prometheus metrics for Celery workers in Flower project and we use this wonderful library :) - thx for your work on it!

One issue I came across is lack of direct access to metric's labelnames and labelvalues.

I think easiest is to give you an example how it is used/why it is needed.

We have multiple celery workers in a kubernetes cluster, they are monitored by Flower which generates Prometheus metrics.
Now the pods in k8s can die or be replaced if a new release of our software is deployed.

The problem with that then is that the final reading for each metric for that pod is retained forever and it shows in grafana forever.

We want to remove any tuples of label values which contain a celery worker name that is deemed offline/dead.

I used a semi-hack (https://github.com/mher/flower/pull/1135) and get those label values through metric.collect(), and from that iterate over samples and the labels in them and then if they contain an offline worker I remove the whole tuple from a given metric by calling metric.remove(*labelvalues_containg_dead_worker)

There could be a better way - access metric._metric.keys() in a thread safe and read-only way.

So I propose to add to MetricWrapperBase a read only property, say all_label_values (name to be improved, suggestions welcome :))

@property
def all_label_values(self) -> List[Tuple[str, ...]]:
    with self._lock:
        all_label_values = list(self._metric.keys().copy())

   return all_label_values

I think I am also badly missing access to actual labelnames set for the given metric to know at which position a label with a given name will be in the labelvalues.

Could kill 2 birds with one stone then and add:

@property
def labelnames(self) -> Tuple[str, ...]:
    return self._labelnames

Please let me know if such a change is acceptable and if I am missing any nuances.

If you think you would be ok merging it I will make a PR with tests soon.

Cheers,

Tom

Beginne mit MetricWrapperBase und prüfe das im Issue beschriebene Verhalten von _metric, _labelnames, _lock, collect() und remove(). Definiere den öffentlichen schreibgeschützten Zugriff auf die Labelnamen und Label-Wert-Tupel und füge anschließend Tests hinzu, die die bereitgestellten Werte und das threadsichere Leseverhalten bestätigen.