Rename to reads_memory_consumption and drop the "active" from the
description as well. This metric tracks the memory consumption of all
reads: active or inactive. We don't even currently have a way to track
the memory consumption of only active reads.
Drop the part of the description which explains the interaction with
other metrics: this part is outdated and the new interactions are much
more complicated, no way to explain in a metric description.
Also ask the semaphore to calculate the memory amount, instead of doing
it in the metric itself.
dcd2deb5af