Support warmup aggression values smaller than 1 in DestinationRule

[arnogeurts]

Describe the feature request

With #3215 the aggression and min_weight_percent settings are added to the warmup of the DestinationRule. However, in that feature the aggression is restricted to values in the range [1, +∞), whereas Envoy supports values in the range [0, +∞). This feature request adds support for the full range of the aggression settings in Envoy.

Describe alternatives you've considered

Affected product area (please put an X in all that apply)

[ ] Configuration Infrastructure
[ ] Docs
[ ] Installation
[x] Networking
[ ] Performance and Scalability
[ ] Policies and Telemetry
[ ] Security
[ ] Test and Release
[ ] User Experience

Additional context

[RafalSumislawski]

Envoy requires aggression to be greater than 0, so the range of supported values should be (0, +∞) not [0, +∞).

https://www.envoyproxy.io/docs/envoy/v1.33.0/api-v3/config/cluster/v3/cluster.proto#envoy-v3-api-field-config-cluster-v3-cluster-slowstartconfig-aggression

[hzxuzhonghu]

Is there much diff with 1 and 0.1?

[RafalSumislawski]

Following the formula in envoys documentation https://www.envoyproxy.io/docs/envoy/latest/intro/arch_overview/upstream/load_balancing/slow_start. The difference is that between x (linear) and x^10 (exponential).

With a 60s warmup, and 1000 req/s traffic per pod, at 30th second of the warmup aggression=1 will send ~500req/s to the warming up pod. And agression=0.1 will send ~1 req/s to that pod. (assuming that min_weight doesn't interfere with that)