feat(aggregators.final): Add new tag instead of appending "_final" to fields

[LarsStegman]

Use Case

We have Telegraf parsing a binary message that comes in at 50Hz. We want to write the 50Hz data to an Influx bucket that is kept locally on a remote location. We want to take the last values and write them to a bucket that is replicated to a central location. We also want to keep the same data schema. By adding a tag instead of changing the field name, we can route the down sampled values to a different influx output.

Expected behavior

The field names are the same, but a tag is added to indicate the metrics have passed through the aggregator.

The extra tag can of course be added using tags.

[[aggregators.final]]
  tags = {"?frequency"="low"}
  period = "1s"
  drop_original = false
  rename_fields = false
  output_strategy = "periodic"

Input

data,my=tag my_field=23i
data,my=tag my_field=24i
data,my=tag my_field=25i
data,my=tag my_field=26i
data,my=tag my_field=27i
data,my=tag my_field=28i
data,my=tag my_field=29i
data,my=tag my_field=30i

Output

data,my=tag my_field=20i
data,my=tag my_field=21i
data,my=tag my_field=22i
data,my=tag my_field=23i
data,my=tag my_field=24i
data,my=tag my_field=25i
data,my=tag,?frequency=low my_field=25i
data,my=tag my_field=26i
data,my=tag my_field=27i
data,my=tag my_field=28i
data,my=tag my_field=29i
data,my=tag my_field=30i
data,my=tag,?frequency=low my_field=30i

Actual behavior

Input

data,my=tag my_field=23i
data,my=tag my_field=24i
data,my=tag my_field=25i
data,my=tag my_field=26i
data,my=tag my_field=27i
data,my=tag my_field=28i
data,my=tag my_field=29i
data,my=tag my_field=30i

Output

data,my=tag my_field=20i
data,my=tag my_field=21i
data,my=tag my_field=22i
data,my=tag my_field=23i
data,my=tag my_field=24i
data,my=tag my_field=25i
data,my=tag,?frequency=low my_field_final=25i
data,my=tag my_field=26i
data,my=tag my_field=27i
data,my=tag my_field=28i
data,my=tag my_field=29i
data,my=tag my_field=30i
data,my=tag,?frequency=low my_field_final=30i

Additional info

I am willing to create a PR for this myself.

[LarsStegman]

I was under the impressions that adding tags to an aggregator would also add the tags to the output metrics, but that is apparently not the case.