[COMMON] Disable metrics by default

[COMMON] Disable metrics by default

Similar to how we do not enable tracing by default, metrics should not
be enabled by default either [0]
This requires some adjustments, since it practically was not possible to deploy
ONAP without it:

- add if condition to "common.serviceMonitor" for .Values.global.metrics.enabled
- remove chart-specific if wrappers that do the same
- this allows to toggle metrics monitoring via Prometheus for the whole platform
- without this change, it would not be possible to deploy ONAP without the Prometheus
  Operator

[0] The prometheus backend that is required for this is not part of the ONAP deployment
    and it should thus be a conscious decision to both enable metrics and deploy it

Issue-ID: OOM-3372
Change-Id: I7c585f6f4b77e9e33ea334cc5ee64dcd7f2d164f
Signed-off-by: Fiete Ostkamp <fiete.ostkamp@telekom.de>