ui: Flamegraph expansion handles grouped by label nodes

[manojVivek]

The latest flamegraph expansion logic was broken when the flamegraph is grouped by labels and this fix handles that scenario.

[alwaysmeticulous]

🤖 Meticulous evaluated 50 user flows and took 334 visual snapshots. Meticulous has not yet run on 5271033 of the main branch and so there was nothing to compare against.

If you recently setup Meticulous, this is expected. Meticulous will start reporting comparisons for new pull requests after the next commit to the main branch.

_{Last updated for commit 656a49e. This comment will update as new commits are pushed.}