This is a non functional change which consolidates the various packages
under metrics into the top level package now that the dead code is
removed.
It is a precursor to the removal of Victoria metrics after which all
erigon metrics code will be contained in this single package.
we need to extract this interface from the struct.
i need to also break down the interface more, to better show what parts
the caching is used, move some functions from the cache state to the
underlying.
don't merge
