Add a basic mempool storage Grafana dashboard

[teor2345]

Motivation

I needed to see if the mempool was active to test PR #2862.

This implements part of #2627.

Solution

Create a mempool dashboard that shows:

• storage transaction count
• storage serialized transaction bytes
• syncer downloaded blocks (as a proxy for recent sync lengths, which don't have a usable metric)
• state committed blocks

Review

@conradoplg is working on more metrics.

Reviewer Checklist

• Dashboard makes sense

[teor2345]

Here's what it looks like:

[conradoplg]

Looks good!