feat: update prebuilds dashboard with build time metrics

[ssncferreira]

Description

Update the prebuilds dashboard to compare key metrics between regular and prebuilt workspaces, and to surface prebuilt creation vs claim timings.

Changes

• Remove “Experiment enabled?” panel: prebuilds is no longer experimental: feat: graduate prebuilds to general availability coder#18607
• Switch row repeat to the preset label.
• Add “All-Time: Regular vs Prebuilt” panel: compares total regular workspace creations vs prebuilt workspace claims for each (template, preset).
• Add “All-Time: Prebuilds Usage %” panel: shows the percentage of prebuilds usage when compared with the total number of user workspaces for each (template, preset).
• Add “Workspace Build Latency p50” panel: median (p50) build time in seconds for Regular Workspace Creation, Prebuilt Workspace Creation, and Prebuilt Workspace Claim.
• Add “Workspace Build Latency p95” panel: 95th-percentile (p95) build time in seconds for Regular Workspace Creation, Prebuilt Workspace Creation, and Prebuilt Workspace Claim.

Screenshot

Link

Internal Grafana dashboard: https://grafana.dev.coder.com/goto/WEthVFrHR?orgId=1

[dannykopping]

Looks very good! Left a few thoughts.

[dannykopping]

Curious why you chose to make this dashed?

[ssncferreira]

Just to make a visual distinction between the other panels. But I can change it to Solid.

[dannykopping]

LGTM!