Werk #13847: local: Do not crash upon invalid metric names

In Checkmk 1.6 and earlier the backend would silently sanitize metric names according to the nagios specification. As this is no longer done, the local check plugin crashed upon invalid metric names. As the metric names may be provided by third party components we no longer crash, but issue a warning instead. The measured value will still be shown, and provided levels will be applied.

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