Werk #16121: Fixed CMC crash when sending illegal metrics to InfluxDB

When the CMC tried to send to a metric to an InfluxDB where the metric name was malformed (e.g. starting with an underscore), the CMC crashed. This has been fixed, such metrics are now simply forwarded to the InfluxDB. The latter will probably be a bit unhappy about such metric names, but this can be figured out via its logs, and the corresponding check can be fixed.

Note that in a future Checkmk version, we will probably drop such metrics silently, logging this only for higher log levels to avoid log spam at the default log settings.

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