Add prometheus serializer and use it in prometheus output (#6703)

plugins/serializers/prometheus/README.md

@@ -0,0 +1,68 @@
+# Prometheus
+
+The `prometheus` data format converts metrics into the Prometheus text
+exposition format.  When used with the `prometheus` input, the input should be
+use the `metric_version = 2` option in order to properly round trip metrics.
+
+**Warning**: When generating histogram and summary types, output may
+not be correct if the metric spans multiple batches.  This issue can be
+somewhat, but not fully, mitigated by using outputs that support writing in
+"batch format".  When using histogram and summary types, it is recommended to
+use only the `prometheus_client` output.
+
+## Configuration
+
+```toml
+[[outputs.file]]
+  files = ["stdout"]
+  use_batch_format = true
+
+  ## Include the metric timestamp on each sample.
+  prometheus_export_timestamp = false
+
+  ## Sort prometheus metric families and metric samples.  Useful for
+  ## debugging.
+  prometheus_sort_metrics = false
+
+  ## Output string fields as metric labels; when false string fields are
+  ## discarded.
+  prometheus_string_as_label = false
+
+  ## Data format to output.
+  ## 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 = "prometheus"
+```
+
+### Example
+
+**Example Input**
+```
+cpu,cpu=cpu0 time_guest=8022.6,time_system=26145.98,time_user=92512.89 1574317740000000000
+cpu,cpu=cpu1 time_guest=8097.88,time_system=25223.35,time_user=96519.58 1574317740000000000
+cpu,cpu=cpu2 time_guest=7386.28,time_system=24870.37,time_user=95631.59 1574317740000000000
+cpu,cpu=cpu3 time_guest=7434.19,time_system=24843.71,time_user=93753.88 1574317740000000000
+```
+
+**Example Output**
+```
+# HELP cpu_time_guest Telegraf collected metric
+# TYPE cpu_time_guest counter
+cpu_time_guest{cpu="cpu0"} 9582.54
+cpu_time_guest{cpu="cpu1"} 9660.88
+cpu_time_guest{cpu="cpu2"} 8946.45
+cpu_time_guest{cpu="cpu3"} 9002.31
+# HELP cpu_time_system Telegraf collected metric
+# TYPE cpu_time_system counter
+cpu_time_system{cpu="cpu0"} 28675.47
+cpu_time_system{cpu="cpu1"} 27779.34
+cpu_time_system{cpu="cpu2"} 27406.18
+cpu_time_system{cpu="cpu3"} 27404.97
+# HELP cpu_time_user Telegraf collected metric
+# TYPE cpu_time_user counter
+cpu_time_user{cpu="cpu0"} 99551.84
+cpu_time_user{cpu="cpu1"} 103468.52
+cpu_time_user{cpu="cpu2"} 102591.45
+cpu_time_user{cpu="cpu3"} 100717.05
+```