Collector crashes when a report contains an unsupported source

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If a source type becomes unsupported (i.e. there's no SourceCollector subclass to get the data from the source) and a report still contains a source with the unsupported source type, the collector will crash. The frontend doesn't have his problem, it will simply ignore unknown source types.

Possible solutions:

• Remove unsupported source types from reports during start-up of the server
• Replace unsupported source types with a "unsupported source" type during start-up of the server
• Have the /internal-api/v3/metrics endpoint filter sources that are unsupported
• Have the collector check that source types are supported before trying to instantiate a collector class for that type