Added speaker monitor service labels

[huseyinbabal]

Is this a BUG FIX or a FEATURE ?:
/kind bug

What this PR does / why we need it:
Fixes #2389

Special notes for your reviewer:
You can verify the rendered template for metallb-spekaer-monitor-service as follows;

1. Render helm chart

helm template metallb  --set prometheus.serviceMonitor.enabled=true --set prometheus.serviceAccount=prometheus-monitoring-kube-prometheus --set prometheus.namespace=monitoring > /tmp/metallb-new.yaml

2. Check /tmp/metallb-new.yaml file for metallb-speaker-monitor-service if it contains label app.kubernetes.io/name with value metallb

Release note:

TODO