Add {find,Path} and {select,Pat} as subscriptions

When defining subscribers, rather than providing an exometer entry key, this PR adds the options to either provide `{find, Path}`, or `{select, Pattern}`, which will cause the reporter to periodically run either `exometer:find_entries(Path)` or `exometer:select(Pattern)` and report the found entries.

If no objects are found and `RetryFailedMetrics = false`, the reporter will not try again. It would probably be normal to use `RetryFailedMetrics = true`, so perhaps this should be the default...