fix(grafana): exclude PRs without a metrics row from PR time panels

[chenwei791129]

Summary

The five PR time panels (PR Cycle / Coding / Pickup / Review / Deploy Time) of Engineering Throughput and Cycle
Time and its Team View take merged PRs from pull_requests, LEFT JOIN project_pr_metrics, and
coalesce(..., 0) the metric. A merged PR without a project_pr_metrics row was therefore averaged in as
0 hours. For a project without DORA enabled, every panel showed a flat 0 h.

This PR changes LEFT JOIN project_pr_metrics to JOIN in those 20 queries (2 dashboards × 5 panels ×
MySQL/PostgreSQL). Nothing else in the SQL changes.

Case	Before	After
Some merged PRs have no row	Averaged in as 0 h, pulling the average down	Excluded
No PR has a row (e.g. DORA disabled)	Flat 0 h	No data
Every merged PR has a row (DORA enabled)	—	Unchanged

• coalesce(..., 0) is kept on purpose. A row can still have a NULL coding, pickup, review or deploy time, and
  those values are still treated as 0. All five panels still average over the same set of PRs.
• The PostgreSQL files are edited by hand. For the main dashboard, the result is identical to running
  grafana/scripts/convert-mysql-to-postgresql.py on the updated MySQL file. The PostgreSQL team view already
  differs from the converter output (it uses ${team1:csv} instead of ${team1:singlequote}), so regenerating
  it would undo those edits.

Does this close any open issues?

Closes #9225

Screenshots

No layout changes. Query results are shown under Tests.

Other Information

Not changed here: the join matches on pr.id only, while the primary key of project_pr_metrics is
(id, project_name). Every dashboard that reads this table joins the same way.

Tests

There are no automated tests for dashboard SQL. I ran the 10 panel queries from main and from this branch
against MySQL 8 and PostgreSQL 17 with synthetic data, replacing the Grafana macros with literal values:

Project	Data	main	This PR
p	2 PRs with rows (cycle time 14 h and 10 h), 1 PR without	PR Cycle Time 8 h	12 h
q	Merged PR, no rows (DORA disabled)	0 h on all 10 panels	No rows (No data)
full	Every PR has a row	—	Same as main on all 10 panels

The results are the same on MySQL and PostgreSQL, for both dashboards. All four JSON files parse.