ci: increase otelspan-add-tags SLO (#15258)

## Description

Seems to be flaky and just barely failing on a few PRs. Not sure when
this flakiness was introduced.

## Testing

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## Risks

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-->

## Additional Notes

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Author: brettlangdon

.gitlab/benchmarks/bp-runner.microbenchmarks.fail-on-breach.yml

@@ -802,7 +802,7 @@ experiments:
               - max_rss_usage < 47.50 MB
           - name: otelspan-add-tags
             thresholds:
-              - execution_time < 314.00 ms
+              - execution_time < 321.00 ms
               - max_rss_usage < 47.50 MB
           - name: otelspan-get-context
             thresholds: