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/net_test.go

@@ -23,5 +23,5 @@ func BenchmarkNetCollector(b *testing.B) {
 	// Whitelist is not set in testing context (kingpin flags not parsed), causing the collector to skip all interfaces.
 	localNicWhitelist := ".+"
 	nicWhitelist = &localNicWhitelist
-	benchmarkCollector(b, "net", NewNetworkCollector)
+	benchmarkCollector(b, "net", newNetworkCollector)
 }