[26.0] Use 1-based step index in invocation metric UI

[mvdbeek]

Fixes #21784

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[ahmedhamidawan]

Tested locally, this fixes the indexing, thank you!

[jmchilton]

We should probably have library functionality somewhere that is used between different components to render displayed index from the index in the API. I prefer "use this abstraction" to "add one anywhere we display it". Maybe too much to ask for a bug fix though.

[mvdbeek]

Do we use the nested step index anywhere else ? WES, but it doesn't have a frontend ?