exif-read-tool/main.go: Log size of EXIF blob

Remember that this is typically from the EXIF signature to the end of
the file.

v2/exif-read-tool/main.go

@@ -97,6 +97,8 @@ func main() {
 		log.Panic(err)
 	}
 
+	mainLogger.Debugf(nil, "EXIF blob is (%d) bytes.", len(rawExif))
+
 	// Run the parse.
 
 	im := exif.NewIfdMappingWithStandard()