Is your feature request related to a problem? Please describe.

AWS now provide a new metadata endpoint to (potentially) pre-warn of likely spot interruption on instances. Metrics on these rebalance recommendations along with the time they were generated are likely to be useful for cluster operators.

Describe the solution you'd like to see

The spot termination exporter to (potentially optionally) expose a number of new metrics:

aws_instance_metadata_service_events_available     Metadata service events endpoint available
aws_instance_rebalance_recommended                 Instance rebalance is recommended
aws_instance_rebalance_recommended_at              Unix epoch rebalance recommendation was exposed at

Describe alternatives you've considered

A completely separate component scraping the relevant metadata endpoint (as it differs from the already scraped spot termination endpoint.) However this would result in running another daemonset alongside the existing one.

Additional context

I've already done most of the work to perform this scraping and metrics exposition on an internal fork of the project, happy to raise the PR to add this functionality to the wider project.