feat: add upload/download by group to metrics

[tyler-french]

This is helpful information to see for changes over time, and unexpected traffic patterns.

We can see similar information for executions, and its useful to correlate traffic from the cache and the executors.

Adding to the "Internal" section as this is likely not helpful to others—we can move it to the "remote cache" section if others want it.

[Copilot]

Pull request overview

This PR adds per-organization (group) tracking to remote cache upload and download metrics by adding the group_id label to cache size metrics and creating corresponding Grafana dashboard panels. This enables monitoring of cache usage patterns on a per-organization basis, which is valuable for understanding usage trends and identifying potential issues with specific groups.

Changes:

• Added GroupID label to CacheUploadSizeBytes and CacheDownloadSizeBytes histogram metrics
• Updated metric recording in hit tracker to include group IDs when observing cache transfer sizes
• Added two new Grafana dashboard panels showing download and upload throughput broken down by group ID

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
server/metrics/metrics.go	Added GroupID to label lists for CacheDownloadSizeBytes and CacheUploadSizeBytes metrics
server/remote_cache/hit_tracker/hit_tracker.go	Updated metric observation calls to include groupID in the label sets for cache upload and download size metrics
tools/metrics/grafana/dashboards/buildbuddy.json	Added two new timeseries panels for visualizing download and upload throughput per group

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