Fix otel-export Lux metric ordering

[robacourt]

Issue

• fix(telemetry): align metric exports with emitted events #4201 added new metric assertions to otel-export.lux, but it placed them in a later collector block than the one where those metrics are actually exported.
• In the failing run, electric.plug.serve_shape.* and the related storage metrics were already printed by the OTEL collector before the final Lux block started waiting for them.
• That meant Lux timed out even though the metrics were present in the collector log.
• An earlier fragility made this worse: the loose ?Span #[0-9]+ checks could match arbitrary earlier spans and advance the collector cursor past the export window we actually cared about.

The Fix

• Move each metric assertion into the OTEL export window where that metric actually appears.
• Stop matching arbitrary Span #N lines and instead anchor directly on shape_snapshot.execute_for_shape plus the expected shape.query_reason.
• Keep the test aligned with the real collector output order, instead of assuming the metrics will still be visible in a later block.

Implementation

• move the OTEL metric assertions to the export windows where the metrics actually appear
• stop matching arbitrary Span #N lines before shape_snapshot.execute_for_shape
• keep the test focused on Lux ordering rather than sync-service behavior

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
3512	1	3511	27

View the top 1 failed test(s) by shortest run time

Elixir.Electric.ShapeCacheTest::test get_or_create_shape_handle/2 against real db crashes when initial snapshot query fails to return data quickly enough

Stack Traces | 0s run time

18) test get_or_create_shape_handle/2 against real db crashes when initial snapshot query fails to return data quickly enough (Electric.ShapeCacheTest)
     test/electric/shape_cache_test.exs:507
     ** (EXIT from #PID<0.17815.0>) killed

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