chore: Ensure collector build funcs are private

Collector builder functions are only used internally in the `collector`
package, and shouldn't needlessly be exposed as part of the package API
to downstream clients.

Signed-off-by: Ben Reedy <breed808@breed808.com>

collector/logical_disk_test.go

@@ -9,5 +9,5 @@ func BenchmarkLogicalDiskCollector(b *testing.B) {
 	localVolumeWhitelist := ".+"
 	volumeWhitelist = &localVolumeWhitelist
 
-	benchmarkCollector(b, "logical_disk", NewLogicalDiskCollector)
+	benchmarkCollector(b, "logical_disk", newLogicalDiskCollector)
 }