Relevant telegraf.conf:

[[processors.unpivot]]
  [processors.unpivot.tagpass]
    cpu = ["cpu0"]
  tag_key = "name"
  value_key = "value"

System info:

Telegraf version 1.15.2
Docker 19.03.12 running on Debian Linux version 10.5

Docker

Steps to reproduce:

Use the unpivot processor as illustrated in the documentation, but add a tagpass filter to filter for measurements with tag cpu=cpu0.

Expected behavior:

The fields in the cpu measurement should be unpivoted, i.e., there should be a new measurement for each field, tagged with the field name and having exactly one metric.

Actual behavior:

The measurement is dropped, with the following debug info:

2020-08-27 15:53:19 | 2020-08-27T13:53:19Z D! [serializers.influx] could not serialize field "": invalid field key; discarding field

Additional info:

The unpivot processor works fine, if there are no filters given, and also if the namepass filter is given.