rpk: Improve generated Grafana dashboard with public_metrics

[r-vasquez]

Description

Follow up for: #6165 (comment) to improve the generated grafana dashboard with public_metrics:

1. Refactor the dashboard to include:

This is an example of an ops dashboard that we've produced in CS which covers some of the things we'd be encouraging people to monitor. Unfortunately, it seems that some of these things aren't available in public_metrics, so we may need a separate issue to add those things.
List as follows

1. Nodes Up
2. Uptime
3. No. Partitions
4. No. Topics
5. Leadership transfer rate (not present)
6. Under replicated partitions
7. Leaderless partitions
8. CPU Utilisation (not present) -> Check additional info
9. Allocated Memory
10. Leadership balance
11. Currently active connections (not present)
12. Cluster info (build numbers, versions, etc) (not present)
13. Produce latency
14. Consumer latency
15. Storage bytes written (not present)
16. Storage bytes read (not present)
17. Network bytes received
18. Network bytes sent
19. Under-replicated partitions (by topic) (not present) -> Check additional info
20. Leaderless partitions (list)
21. Under replicated partitions by cluster (not present) -> Can be derived from redpanda_kafka_under_replicated_replicas
22. Number of groups for which a node is a leader
23. Partition leadership per broker

2. Replace Memory and Storage Section:

Replace the panels in the storage section with two new panels that display the ratio of disk available:
Disk Usage per Broker (the percentage of disk currently in use): 1 - (redpanda_storage_disk_free_bytes / redpanda_storage_disk_total_bytes)

Replace the memory section with: Memory Usage per Broker (the percentage of memory currently in use): redpanda_memory_allocated_memory / (redpanda_memory_free_memory + redpanda_memory_allocated_memory)

3. Aggregate the rest proxy and schema registry errors by redpanda_status

The queries should change like this: sum(...) by ($aggr_criteria, redpanda_status). Note the new label we aggregate by.

Additional Info:

#6165 (comment)

This will require rpk to handle /public_metrics differently from how we handle /metrics to keep backompat of the old dashboard.

[r-vasquez]

Fixed by: #9662