* Adds back indexer metrics. Uses config values instead of hardcoded ones.
* cast to int64
* remove unused func
* Index metrics impl doesn't depend on Bleve. Adds a TotalDocs func to SearchBackend interface.
* adds config setting for index_min_count
* rename arg
* rename metric label to namespace instead of slug
* adds default "do nothing" case to satisfy linter
* moves bleve index metrics to search package
* make bleve backend private, dont need to pass in prom reg
* imports
* adds bleve metrics to resource package to avoid circular deps
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