Add carbon2 serializer (#5345)

plugins/serializers/carbon2/README.md

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+# Carbon2
+
+The `carbon2` serializer translates the Telegraf metric format to the [Carbon2 format](http://metrics20.org/implementations/).
+
+### Configuration
+
+```toml
+[[outputs.file]]
+  ## Files to write to, "stdout" is a specially handled file.
+  files = ["stdout", "/tmp/metrics.out"]
+
+  ## 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_OUTPUT.md
+  data_format = "carbon2"
+```
+
+Standard form:
+```
+metric=name field=field_1 host=foo  30 1234567890
+metric=name field=field_2 host=foo  4 1234567890
+metric=name field=field_N host=foo  59 1234567890
+```
+
+### Metrics
+
+The serializer converts the metrics by creating `intrinsic_tags` using the combination of metric name and fields.  So, if one Telegraf metric has 4 fields, the `carbon2` output will be 4 separate metrics. There will be a `metric` tag that represents the name of the metric and a `field` tag to represent the field.
+
+### Example
+
+If we take the following InfluxDB Line Protocol:
+
+```
+weather,location=us-midwest,season=summer temperature=82,wind=100 1234567890
+```
+
+after serializing in Carbon2, the result would be:
+
+```
+metric=weather field=temperature location=us-midwest season=summer  82 1234567890
+metric=weather field=wind location=us-midwest season=summer  100 1234567890
+```
+
+### Fields and Tags with spaces
+When a field key or tag key/value have spaces, spaces will be replaced with `_`.
+
+### Tags with empty values
+When a tag's value is empty, it will be replaced with `null`