Allow gLiveView to still provide Mitril Signer Metrics when decoupled from node.

[TrevorBenson]

Describe the Feature
When the node and the signer run in the same pod (containerized environments / k8s) they share the same IP / Loopback(s) (127.0.0.1 etc.). The current logic for setup of METRICS_SERVER_IP supports this nicely. However once the mithril signer runs in its own pod it does not share a loopback and the current METRICS_SERVER_IP value is 0.0.0.0, which makes gLiveView presume it is locally available.

To allow splitting the node and the signer into separate pods a new MITHRIL_SIGNER_HOST variable could be set/defined in env and used by gLiveView. When defined it would override the METICS_SERVER_IP, when undefined it would inherit the METRICS_SERVER_IP if it is defined.

Expected behavior

• The MITHRIL_SIGNER_HOST variable provides a way to set a hostname or an IP address that can be used for gLiveView to still provide metrics for the Mithril Signer.
• The existing behavior of gLiveView for Mithril metrics should remain unchanged.

Additional context
While mostly applicable to pod based container environments (k8s, containerd, podman) it would also apply to SPO's who intentionally decouple a Mithril Signer onto a separate host w/ the required keys, but still would like to see metrics in gLiveView dashboard.