PR Dashboard: Allow visibility into pre-specified labels

[drewrelmas]

We just enabled the PR Dashboard in otel-arrow, and it works beautifully!

open-telemetry/otel-arrow#3493

One thing we have done to help reviewers easily triage open PRs is applying labels, specifically in:

• https://github.com/open-telemetry/otel-arrow/blob/main/.github/workflows/pr-size-label.yml (label size/{XS|S|M|L|XL} based on changed line count)
• https://github.com/open-telemetry/otel-arrow/blob/main/.github/workflows/label-breaking-changes.yml (label breaking change based on user-submitted changelog)

It would be great if we could display certain labels on the dashboard as well to help inform the priority of reviews.

Obviously do not want all labels to be displayed, only certain ones that each repo maintainer deems useful for their own repository.