[MONIT-4003] metrics: add default node selector option

This chart includes some components that are composed of a single pod, like: prometheus and fluentbit to collect and forward metrics or kube state to expose internal kubernetes api metrics.

This commit adds a new configuration field (merics.defaultNodeSelector) that allows to set a common nodeSelector for all those components. In case that any of those components adds a nodeSelector that one will have more priority.