Parse Indexer Unavailable Errors to emit unavailable_indexers metric

[rtrox]

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Describe the solution you'd like:

Prowlarr, interestingly, does not consider indexers which have been marked "unavailable" as "disabled", so they don't increment the disabled metric (It only counts indexers which were manually disabled). Unavailable indexers seems like a metric that will commonly want to be alerted on, but the only place it shows up is as a health issue (of source IndexerLongTermStatusCheck). We should consider special casing this error type to emit an unavailable indexer metric. The actual logic isn't particularly difficulty, but system health issues are collected by a common collector, so we probably need to think a little bit about where and how to collect this and emit it as another metric.

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