Performance data values parsing problem: inf, -inf

[RincewindsHat]

Describe the bug

Some special performance data values like inf and -inf are parsed without hesitation by Icinga2
although they are probably in every case caused by a faulty plugin.
Since the performance data value in Icinga2 is internally a double as far as I could deduct,
the value is "correctly" parsed (by strtold in the end if I am correct), but the resulting DBL_MIN or DBL_MAX can later cause
errors in Performance data writers (InfluxDB(2)) which do not accept
inf or -inf as numbers.

To Reproduce

1. Provide a check result with a performance data value foo=inf to Icinga2
2. get the

   [2024-06-07 10:37:59 +0200] warning/Influxdb2Writer: Unexpected response code: Bad Request
   [2024-06-07 10:37:59 +0200] warning/Influxdb2Writer: Unexpected Content-Type: application/json; charset=utf-8
   

   message in /var/log/icinga2/icinga2.log

Expected behavior

Reject performance data values which are not concrete numbers and write a warning/error message to the log, so one can debug the monitoring plugin.

Your Environment

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

• Version used (icinga2 --version): 2.14.2-1 (from packages.icinga.com)
• Operating System and version: Debian bookworm
• Enabled features (icinga2 feature list): InfluxDB2Writer

[RincewindsHat]

ref/NC/820245