Werk #13468: mrpe: Do not crash upon invalid metrics

In Checkmk versions prior to 2.0, invalid performance data strings passed by the MRPE plugin have been ignored.

In Checkmk 2.0 the parsing would find those invalid strings, and crash upon them. This is generally the desired behaviour (see the documentation for a rationale).

However, since MRPE comprises a lot of different plugins, an all-or-nothing approach is rather inconvenient. We now parse everything as good as we can, and issue an UNKNOWN state for the affected service.

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