Add lux coverage for the shape_db pool checkout queue_time metric

Confirms the metric name is exported in plain ASCII.

Author: alco

integration-tests/tests/otel-export.lux

@@ -114,6 +114,11 @@
   [invoke grep_otel_collector_output "Metric electric\.postgres\.replication\.slot_confirmed_flush_lsn_lag .*stack_id=single_stack"]
   [invoke grep_otel_collector_output "Metric electric\.postgres\.replication\.slot_retained_wal_size .*stack_id=single_stack"]
 
+  # The metric name must be plain ASCII (no "µs"/"μs" suffix) so that ingestion
+  # into backends like Mimir/Prometheus, which reject non-ASCII characters in
+  # metric names, doesn't fail.
+  [invoke grep_otel_collector_output "Metric electric\.shape_db\.pool\.checkout\.queue_time_us .*stack_id=single_stack"]
+
   # Regression check: These metrics haven't been exporting until recently
   [invoke grep_otel_collector_output "Metric electric\.plug\.serve_shape\.bytes .*stack_id=single_stack"]
   [invoke grep_otel_collector_output "Metric electric\.plug\.serve_shape\.count .*stack_id=single_stack"]