Enhancement: publish number of files into prometheus (too_old_files_count)

[matthewdarwin]

copy from: streamingfast/merger#32

Publish a Prometheus metric that exposes the too_old_files_count information

2022-01-10T22:25:23.030Z (merger) retrieved list of files (merger/merger.go:179){"too_old_files_count": 0, "too_old_files_count": 884}