this replaces the enricher implementation to improve its enrichment
performance. it drops the enrichment cache and it also doesn't filter
metrics based on the meta records at the enrichment stage anymore.
instead it is now building a map from which it can lookup the metric
keys and resolve them into meta records, from which it then gets the
meta tags.
especially in scenarios where there is a large number of meta records in
the index this performs much better than the old implementation.

this has the purpose of improving the addMetric performance when a large
number of metrics gets added to the index concurrently. previously each
of them would have been checked against the filter requirements of each
existing meta record, due to how we now process them in batches this
process is more efficient. instead of checking each new metric one by
one against each meta record criteria, we're now building a small
temporary index out of all the added metrics in the buffer, then we run
each meta record as a query on that small index. this change improves
the addMetric event processing performance by a huge factor in
situations where a lot of metrics get added at once.