getrusage for non-child process?

Question

The man page for getrusage lists several modes:

   RUSAGE_SELF
          Return  resource usage statistics for the calling process, which is the sum of resources used by all threads
          in the process.

   RUSAGE_CHILDREN
          Return resource usage statistics for all children of the calling  process  that  have  terminated  and  been
          waited  for.  These statistics will include the resources used by grandchildren, and further removed descen‐
          dants, if all of the intervening descendants waited on their terminated children.

   RUSAGE_THREAD (since Linux 2.6.26)
          Return resource usage statistics for the calling thread.  The _GNU_SOURCE feature test macro must be defined
          (before including any header file) in order to obtain the definition of this constant from <sys/resource.h>.

However none of these seem to allow getting data for an unrelated process, only for children. I'd prefer to keep getrusage calls out of my critical path processes, and I have an existing monitor process for doing this kind of work, but the monitor process does not actually spawn the others, so it's not the parent of them. Even if I changed it to be the parent, RUSAGE_CHILDREN requires the child to have terminated! I'd like to be monitoring some of these stats in realtime, these are long lived processes. Is there an alternative way to get the same data? Do I have to repeatedly read /proc files?