Degraded health state

[Tratcher]

Currently a destination's health may be marked as Unknown, Unhealthy, or Healthy.

ASP.NET Core also supports a Degraded state:
https://github.com/dotnet/aspnetcore/blob/a31f34e8a1618210dc47675d667180765203cdfb/src/HealthChecks/Abstractions/src/HealthStatus.cs#L30

This might be useful when filtering and prioritizing destinations for traffic. E.g. a degraded destination may be sporadically returning errors, but that's still better than an unhealthy endpoint that's unreachable.