Decode a sweep membership from an instance's tags, or undefined if the
instance carries no spawn:sweep-id (i.e. it's not part of a sweep). Mirrors
the sweep/param branch of the Go describe path (pkg/aws/client.go). Malformed
numeric tags fall back to 0 rather than being fatal.
Decode a sweep membership from an instance's tags, or undefined if the instance carries no spawn:sweep-id (i.e. it's not part of a sweep). Mirrors the sweep/param branch of the Go describe path (pkg/aws/client.go). Malformed numeric tags fall back to 0 rather than being fatal.