InfluxdbWriter chokes on perfdata "U"

Describe the bug

I am getting a lot of warnings in my icinga2 logs like this:

warning/InfluxdbWriter: Ignoring invalid perfdata for checkable 'dev-bas1-pos-v01.sf-rz.de!postgres_patroni_dev_01_tst_longest_query' and command 'check_pgactivity' with value: 'testdb max'=Us;60;90

Looking at https://nagios-plugins.org/doc/guidelines.html#AEN200, the value "U" is perfectly legitimate here

To Reproduce

1. set up a check which will return "U" as value in the performancedata (check_pgactivity works fine with the longest_query)
2. run the check and look into the logs

Expected behavior

I would love to turn off those warnings in the icinga logs (only for value U) or have InfluxWriter insert something like null instead of failing/complaining.

Your Environment

Include as many relevant details about the environment you experienced the problem in

• Version used (icinga2 --version): 2.13.2-1
• Operating System and version: RHEL 7.9
• Enabled features (icinga2 feature list): api checker ido-pgsql influxdb mainlog notification

Start at the InfluxdbWriter handling of performance data and reproduce the warning with a check_pgactivity result containing the value "U". Verify the behavior through the icinga2 logs and confirm that this valid value no longer produces a warning or is represented as null in InfluxDB.