[API Gateway] HTTPRoutes with weights that don't divide evenly into 10000

[andrewstucki]

Overview of the Issue

Under the hood we do calculations based on the passed in weight for HTTPRoute services to normalize them into a percentage, which the virtual service splitters end up translating internally to envoy xDS, scaled to its required total weight of 10000 per-route.

consul/agent/consul/discoverychain/gateway_httproute.go

Lines 137 to 141 in 73b9b40

	weightPercentage := float32(service.Weight) / float32(totalWeight)
	split := structs.ServiceSplit{
	RequestHeaders: modifier,
	Weight: weightPercentage * 100,
	}

When we provide weights that, when summed, don't divide evenly into 10000, we get errors due to errors in float rounding and envoy complains that it requires a total weight of 10000 when doing weighted load-balancing.

Reproduction Steps

Create and attach an HTTPRoute with weights for a set of services that don't divide evenly into 10000, such as:

Rules = [
  {
    Services = [
      {
        Name = "service-one"
        Weight = 2
      },
      {
        Name = "service-two"
        Weight = 1
      }
    ]
  }
]

Here's a simple go snippet of the rounding behavior we're hitting. We should ensure that we handle rounding errors by doing something like adding whatever offset necessary to the highest weighted service in order to normalize all weight percentages to exactly equal 100 (or 10000 when converted to xDS).