das: Better metric for showing sampled range now that SamplingWindow is enabled

[renaynay]

Right now, sampled chain head refers to the tip of the sampled chain which, while accurate, does not reflect that the light node may not have sampled everything before the SamplingWindow.

There should be some sort of metric that reflects where the light node began sampling potentially - or some way to know whether a light node has actually triggered a sampling job over block at height N.

Needs to be fleshed out a bit more.

(from discussions with @jcstein)

[jcstein]

I think #3196 should accomplish part of this, but may be wrong. cc @ramin

[ramin]

@jcstein this wont quite be solved by #3196, this is issue is regarding telemetry of metrics we'd instrument in the code and which node operators could optionally collect and visualize to show metrics about the operation of a running node

[renaynay]

Now that #3194 is resolved, this one needs to be as well.

[walldiss]

It should instead be a metric on the pruner. DASer is unaware of where or whether the latest pruned height is. In fact, it already exists as prnr_last_pruned (the pruner's highest pruned height). Therefore, the range of stored samples can be easily determined between the DASer head and this metric.

[renaynay]

@walldiss we have metrics on pruner (prune head) so does this resolve the issue here?