fix(telemetry): stop the dashboard lint passing when it checked nothing

Run with no arguments the script iterated an empty list, found no
violations and printed "OK: 0 dashboard(s) passed" with exit 0 -- a clean
bill of health for reading no files, indistinguishable from a real pass.

A bare run now defaults to every dashboard beside the script, and a run
that still ends up with nothing to check exits 2 rather than reporting
success. Passing paths explicitly behaves as before.

docker/telemetry/grafana/dashboards/validate_dashboards.py

@@ -2,6 +2,7 @@
 """Dashboard lint: cumulative metrics must be rate()-wrapped; tier filters present."""
 
 import json, re, sys
+from pathlib import Path
 
 # Prometheus gauges that hold a CUMULATIVE total -> must be rate()/increase()-wrapped.
 CUMULATIVE_PREFIXES = (
@@ -111,8 +112,23 @@ def check(path, forbid_5m):
 
 
 def main():
+    """Validate the dashboards named on the command line, or all of them.
+
+    Exit status: 0 every dashboard passed, 1 violations were found, 2 there was
+    nothing to validate. The last is an error rather than a pass: reporting
+    success without having read a single dashboard is indistinguishable from a
+    clean run, so a caller that mis-spells a path gets a green light for work
+    that never happened.
+    """
     args = [a for a in sys.argv[1:] if not a.startswith("--")]
     forbid_5m = "--no-5m" in sys.argv
+    if not args:
+        # Default to every dashboard beside this script, so a bare run checks
+        # the whole set instead of iterating an empty list.
+        args = sorted(str(p) for p in Path(__file__).resolve().parent.glob("*.json"))
+    if not args:
+        print("ERROR: no dashboard JSON files to validate", file=sys.stderr)
+        sys.exit(2)
     all_errs = []
     for path in args:
         all_errs += check(path, forbid_5m)