Measure the fraction of Filecoin we are checking

[bajtos]

Based on the discussion in https://filecoinproject.slack.com/archives/C03PQG6UT2B/p1696624329337349, let's add a new Grafana chart to keep track of the following metrics:

• How many Filecoin deals did we test in the given SPARK round?
• A proxy metric: how many unique (PayloadCID, SP) pairs did we test in the given SPARK round?

Using that data, we should be able to answer the following questions:

• How many Filecoin deals (or (PayloadCID, SP) pairs) can we test every day?
• What are the trends? Is this number increasing, decreasing, staying the same?

Ultimately, we want to answer the following question:

• % of network sampled per day

We should also keep track of % of dropped/failed retrieval deals. That's where we want to make an impact - make more deals retrievable.