Add zpool_influxdb command

A zpool_influxdb command is introduced to ease the collection
of zpool statistics into the InfluxDB time-series database.
Examples are given on how to integrate with the telegraf
statistics aggregator, a companion to influxdb.

Finally, a grafana dashboard template is included to show
how pool latency distributions can be visualized in a
ZFS + telegraf + influxdb  + grafana environment.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Richard Elling <Richard.Elling@RichardElling.com>
Closes #10786

cmd/zpool_influxdb/telegraf.d/README.md

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+This directory contains sample telegraf configurations for
+adding `zpool_influxdb` as an input plugin. Depending on your
+telegraf configuration, the installation can be as simple as
+copying one of these to the `/etc/telegraf/telegraf.d` directory
+and restarting `systemctl restart telegraf`
+
+See the telegraf docs for more information on input plugins.

cmd/zpool_influxdb/telegraf.d/exec_zpool_influxdb.conf

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+# # Read metrics from zpool_influxdb
+[[inputs.exec]]
+#   ## default installation location for zpool_influxdb command
+  commands = ["/usr/local/bin/zpool_influxdb"]
+#   ## Timeout for each command to complete.
+#   timeout = "5s"
+#
+#   ## measurement name suffix (for separating different commands)
+#   name_suffix = "_mycollector"
+#
+#   ## Data format to consume.
+#   ## Each data format has its own unique set of configuration options, read
+#   ## more about them here:
+#   ## https://github.com/influxdata/telegraf/blob/master/docs/DATA_FORMATS_INPUT.md
+  data_format = "influx"

cmd/zpool_influxdb/telegraf.d/execd_zpool_influxdb.conf

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+# # Read metrics from zpool_influxdb
+[[inputs.execd]]
+#   ## default installation location for zpool_influxdb command
+  command = ["/usr/local/bin/zpool_influxdb", "--execd"]
+
+    ## Define how the process is signaled on each collection interval.
+    ## Valid values are:
+    ##   "none"    : Do not signal anything. (Recommended for service inputs)
+    ##               The process must output metrics by itself.
+    ##   "STDIN"   : Send a newline on STDIN. (Recommended for gather inputs)
+    ##   "SIGHUP"  : Send a HUP signal. Not available on Windows. (not recommended)
+    ##   "SIGUSR1" : Send a USR1 signal. Not available on Windows.
+    ##   "SIGUSR2" : Send a USR2 signal. Not available on Windows.
+  signal = "STDIN"
+
+  ## Delay before the process is restarted after an unexpected termination
+  restart_delay = "10s"
+
+    ## Data format to consume.
+    ## Each data format has its own unique set of configuration options, read
+    ## more about them here:
+    ## https://github.com/influxdata/telegraf/blob/master/docs/DATA_FORMATS_INPUT.md
+  data_format = "influx"