temporal-mixin: add Billable Actions section to overview dashboard (#1632)

* temporal-mixin: add Billable Actions section to overview dashboard

Add 9 new panels for monitoring Temporal Cloud billable action costs:
- Billable Actions rate by action type and workflow type (timeseries)
- 30d, 7d, and range estimated total action counts (stat)
- Top 20 billable actions by workflow type (table)
- Action Rate vs. 2x 7d baseline for spike detection (timeseries)
- Heartbeat and Retry ratio gauges for cost optimization

Also cleans up datasource templating for portability by removing
environment-specific defaults and regex filters.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* temporal-mixin: pr review fixes for billable actions dashboard

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* temporal-mixin: use sum_over_time for billable action stat panels

Aligns with the existing Usage & Quotas formula pattern. Each data
point is a 1-minute average rate, so sum_over_time * 60 gives the
actual observed action count rather than an extrapolated estimate.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Update temporal-mixin/dashboards/temporal-overview.json

Co-authored-by: Dominik Süß <dominik@suess.wtf>

* Update temporal-mixin/dashboards/temporal-overview.json

Co-authored-by: Dominik Süß <dominik@suess.wtf>

* Update temporal-mixin/dashboards/temporal-overview.json

Co-authored-by: Dominik Süß <dominik@suess.wtf>

* Update temporal-mixin/dashboards/temporal-overview.json

Co-authored-by: Dominik Süß <dominik@suess.wtf>

* temporal-mixin: fix baseline query and add action/s units

- Fix Action Rate vs. Baseline subquery parse error by restructuring
  to use avg_over_time on the vector selector directly
- Add action/s unit to Billable Actions timeseries panels

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Dominik Süß <dominik@suess.wtf>