Add initial doc for stats collection describing ais-stats usage.

docs/stats_collection.rst

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+Stats Collection Tool
+=====================
+
+Command-line Tool
+-----------------
+``ais-stats`` can be run two ways:
+* ``$ ais-stats -p <PID> [-i]`` will collect stats from a running process.
+  ``-i`` will report immediately rather than wait for the process to exit.
+* ``$ ais-stats <program> [args...]`` will launch ``<program>`` with the
+  provided arguments and report the collected stats when it exits.
+
+Configuration
+-------------
+Collection is controlled by the environment variable ``HIPFILE_STATS_LEVEL``.
+
+===== =================================================
+Value Description
+===== =================================================
+0     Disabled
+1     Basic
+2     Detailed (same as basic; reserved for future use)
+===== =================================================
+
+Stats Collected
+---------------
+* Basic: Bytes read/written on the fastpath backend
+* Basic: Bytes read/written on the fallback backend