Removing/flushing unused dynamic attributes in metric instruments is very important to not leaking memories.

Spec issue: open-telemetry/opentelemetry-specification#1297
Prometheus Suggestion: https://prometheus.io/docs/instrumenting/writing_clientlibs/#labels

Metrics with labels SHOULD support a remove() method with the same signature as labels() that will remove a Child from the metric no longer exporting it, and a clear() method that removes all Children from the metric. These invalidate caching of Children.

Opening this issue to keep track of this problem.