Describe the bug
When creating StageData objects for trace generation, there is no limit on the number of objects created, which can result in unbounded memory consumption.

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• Add documentation label if this issue is related to documentation changes.

To Reproduce
Steps to reproduce the behavior:

1. Create a pipeline that produces thousands of stages.
2. Run the pipeline.
3. Check heap usage data, e.g. by hitting the /monitoring endpoint if using the monitoring plugin.
4. Observe large number of StageData objects.

Expected behavior
The code already throws out stage data when the size of the associated json object exceeds 5000 characters, so we should be short-circuiting StageData creation as well.

Screenshots
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Environment and Versions (please complete the following information):
I'm using Jenkins 6.0.3 and believe the issue was introduced in 6.0.0, in this change #381

Additional context
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