Export in statistics information about whether the node is UP

### Problem

The platform has PlatformStatus, and exports through metrics the status of the platform. For example, there is a platform status of ACTIVE. But ACTIVE is not really meaningful to node operators -- what they really need to know is whether the node is UP or DOWN -- that is, whether the grpc endpoints are UP *and* the platform is ACTIVE. And it should be that the endpoints are DOWN when the platform is *not* ACTIVE.

This should all be made available through stats, so we can keep track of UP/DOWN time, and also so local queries of the prometheus endpoint will report the UP/DOWN status of the node.

### Solution

Keep track of UP/DOWN in `Hedera.java` and update a metric with this information.

### Alternatives

We could have a more generic status, which combines PlatformStatus with UP/DOWN.

Start in Hedera.java, where the issue says node UP/DOWN should be tracked, and inspect the existing PlatformStatus statistics and metrics. Define the metric from whether the gRPC endpoints are up and the platform is ACTIVE, with non-ACTIVE meaning DOWN. Verify that the resulting statistics expose usable UP/DOWN time and local metric queries report the node status.

Written by the indexing model from the issue text.