[Heatmap] Support custom annotations for Heatmap in Lens

[dominiqueclarke]

Is your feature request related to a problem? Please describe. When using a time heatmap, it would be helpful to include annotations along various time buckets.

Upcoming on the roadmap items for the Synthetics/Uptime team include annotations along a heatmap within our new Errors Overview and Error Details pages.

These charts would include a timeline of Uptime down checks, alongside annotations of when specific errors started.

The heatmap itself represents the number of down checks, while the annotations represent when the error started. If a new error is present within a series of down checks, we want to also be able to annotate where the new error started.

Examples

Additionally, all charts in Synthetics should be built as Lens/Exploratory View embeddables, so ideally annotations would be supported for heatmaps

Describe the solution you'd like The ability to add annotations to Heatmaps, particularly as a layer in Lens.

Kibana Cross Issues

• https://github.com/elastic/synthetics-dev/issues/104

Checklist

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• [x] every related Kibana issue is listed under Kibana Cross Issues list
• [x] kibana cross issue tag is associated to the issue if any kibana cross issue is present