Fix metrics chart not rendering (Redis TS 1.8+ QUERYINDEX compatibility)

[mgodwin]

Summary

• Root cause: Redis TimeSeries 1.8+ requires at least one equality filter (label=value) in TS.QUERYINDEX. The previous code used only existence filters (name!= compaction!=), which raises ERR TSDB: please provide at least one matcher. This error was silently caught, leaving @all_series = [] — which caused the controller to fall back to the wrong default compaction (avg instead of sum for counters), produce no chart data, and render a blank section with no feedback.
• Two code paths were broken: the index action (chart rendering) and the labels action (group-by options).
• Dotted metric names (request.count) also failed to route to the labels member endpoint without an explicit constraint.

Changes

File	Change
metrics_controller.rb	index: add .where(compaction: %w[sum avg min max]) to provide required equality filter; labels: use query_index(metric_name) directly (generates name=value equality filter, more efficient)
metrics/index.html.erb	Add explicit "No data for X in the selected time range" message when metric is selected but chart data is empty — previously nothing was rendered
config/routes.rb	Add constraints: { name: /[^\/]+/ } so metric names containing dots route correctly to the labels endpoint
metrics_controller_test.rb	New: 18 tests covering index (summary stats, chart rendering, data format, stacking, explicit params), autocomplete, and labels endpoints
test/dummy/db/schema.rb	Rails version bump artifact (7.1 → 8.1)

Test plan

• Run bin/rails test test/controllers/rails_observatory/metrics_controller_test.rb — all 18 tests pass
• Navigate to /observatory/metrics, click the search field, select a metric, verify the chart renders
• Verify the "No data for X" message appears when selecting a metric with no data in the current time window