Remove timestamp from Metric partition key

[Dav1dde]

Having the timestamp included in the metric partition key hurts the performance of the cardinality limiter. While for aggregation we are only interested in a single bucket, the cardinality limiter is interested in the same metric over a long period of time.

Removing the timestamp from the partition key will (greatly) improve the performance of the in memory cardinality cache by consistently routing unique metrics to the same Relay instance hitting the already populated cache.