client statistics improvements

[jku]

https://github.com/sigstore/rekor-tiles/wiki/Client-statistics and the "clients" dashboard in GCP are useful but potential improvements include:

• Add sum of all requests in last 7d to timestamp.json to the dashboard (to e.g. be able to tell what percentage of traffic is rekor v2 compatible)
• Add number of POST requests (or number of created entries) in last 7d for rekor v1, and (sum of) POST requests to all rekor v2 instances -- so we can compare how this changes once we switch to using rekor v2
• maybe do user-agent analysis of rekor POST request as well -- the current one in rekor Dashboard is a little useless

Note:

• I have trouble using the existing rekor-traffic metric in any widgets like the above: the UI just keeps spinning the progress indicator -- possibly the issue is rekor-traffic metric has too high cardinality terraform-modules#125