Performance logging and counters:

* Tally and duration counters for Job Queue tasks and RPC calls
    optionally rendered by server_info and server_state, and
    optionally printed to a distinct log file.
    - Tally each Job Queue task as it is queued, starts, and
      finishes running. Track total duration queued and running.
    - Tally each RPC call as it starts and either finishes
      successfully or throws an exception. Track total running
      duration for each.
  * Track currently executing Job Queue tasks and RPC methods
    along with durations.
  * Json-formatted performance log file written by a dedicated
    thread, for above-described data.
  * New optional parameter, "counters", for server_info and
    server_state. If set, render Job Queue and RPC call counters
    as well as currently executing tasks.
  * New configuration section, "[perf]", to optionally control
    performance logging to a file.
  * Support optional sub-second periods when rendering human-readable
    time points.

cfg/rippled-example.cfg

@@ -930,6 +930,24 @@
 #     address=192.168.0.95:4201
 #     prefix=my_validator
 #
+# [perf]
+#
+#   Configuration of performance logging. If enabled, write Json-formatted
+#   performance-oriented data periodically to a distinct log file.
+#
+#     "perf_log"      A string specifying the pathname of the performance log
+#                     file. A relative pathname will log relative to the
+#                     configuration directory. Required to enable
+#                     performance logging.
+#
+#     "log_interval"  Integer value for number of seconds between writing
+#                     to performance log. Default 1.
+#
+#   Example:
+#     [perf]
+#     perf_log=/var/log/rippled/perf.log
+#     log_interval=2
+#
 #-------------------------------------------------------------------------------
 #
 # 7. Voting