Add a starlark example showing how to obtain IOPS (influxdata#8996)

plugins/processors/starlark/README.md

@@ -220,6 +220,7 @@ def apply(metric):
 
 - [drop string fields](/plugins/processors/starlark/testdata/drop_string_fields.star) - Drop fields containing string values.
 - [drop fields with unexpected type](/plugins/processors/starlark/testdata/drop_fields_with_unexpected_type.star) - Drop fields containing unexpected value types.
+- [iops](/plugins/processors/starlark/testdata/iops.star) - obtain IOPS (to aggregate, to produce max_iops)
 - [json](/plugins/processors/starlark/testdata/json.star) - an example of processing JSON from a field in a metric
 - [number logic](/plugins/processors/starlark/testdata/number_logic.star) - transform a numerical value to another numerical value
 - [pivot](/plugins/processors/starlark/testdata/pivot.star) - Pivots a key's value to be the key for another key.

plugins/processors/starlark/testdata/iops.star

@@ -0,0 +1,54 @@
+# Example showing how to obtain IOPS (to aggregate, to produce max_iops).  Input can be produced by:
+#
+#[[inputs.diskio]]
+#  alias = "diskio1s"
+#  interval = "1s"
+#  fieldpass = ["reads", "writes"]
+#  name_suffix = "1s"
+#
+# Example Input:
+# diskio1s,host=hostname,name=diska reads=0i,writes=0i 1554079521000000000
+# diskio1s,host=hostname,name=diska reads=0i,writes=0i 1554079522000000000
+# diskio1s,host=hostname,name=diska reads=110i,writes=0i 1554079523000000000
+# diskio1s,host=hostname,name=diska reads=110i,writes=30i 1554079524000000000
+# diskio1s,host=hostname,name=diska reads=160i,writes=70i 1554079525000000000
+#
+# Example Output:
+# diskiops,host=hostname,name=diska readsps=0,writesps=0,iops=0 1554079522000000000
+# diskiops,host=hostname,name=diska readsps=110,writesps=0,iops=110 1554079523000000000
+# diskiops,host=hostname,name=diska readsps=0,writesps=30,iops=30 1554079524000000000
+# diskiops,host=hostname,name=diska readsps=50,writesps=40,iops=90 1554079525000000000
+
+state = { }
+
+def apply(metric):
+    disk_name = metric.tags["name"]
+    # Load from the shared last_state the metric for the disk name
+    last = state.get(disk_name)
+    # Store the deepcopy of the new metric into the shared last_state and assign it to the key "last"
+    # NB: To store a metric into the shared last_state you have to deep copy it
+    state[disk_name] = deepcopy(metric)
+    if last != None:
+        # Create the new metrics
+        diskiops = Metric("diskiops")
+        # Calculate reads/writes per second
+        reads = metric.fields["reads"] - last.fields["reads"]
+        writes = metric.fields["writes"] - last.fields["writes"]
+        io = reads + writes
+        interval_seconds = ( metric.time - last.time ) / 1000000000
+        diskiops.fields["readsps"] = ( reads / interval_seconds )
+        diskiops.fields["writesps"] = ( writes / interval_seconds )
+        diskiops.fields["iops"] = ( io / interval_seconds )
+        diskiops.tags["name"] = disk_name
+        diskiops.tags["host"] = metric.tags["host"]
+        return [diskiops]
+
+# This could be aggregated to obtain max IOPS using:
+#
+# [[aggregators.basicstats]]
+#  namepass = ["diskiops"]
+#  period = "60s"
+#  drop_original = true
+#  stats = ["max"]
+#
+# diskiops,host=hostname,name=diska readsps_max=110,writesps_max=40,iops_max=110 1554079525000000000