Java client incorrectly flushes metric labels to pushgateway when -Dfile.encoding != UTF-8

[fedotxxl]

1. Start application with -Dfile.encoding=cp1251
2. Created counter with label 'Привет мир!'
3. Check :9091/metrics

Looks like the problem is https://github.com/prometheus/client_java/blob/master/simpleclient_pushgateway/src/main/java/io/prometheus/client/exporter/PushGateway.java#L257 :

new BufferedWriter(new OutputStreamWriter(connection.getOutputStream()))

I think encoding should be forced to UTF-8

[brian-brazil]

This should probably be fixed down in TextFormat. Would you like to send a PR?