Add envoy connection balancing.

[hashi-derek]

Description

This PR adds support for the Envoy exact_balance configuration option.

https://cloudnative.to/envoy/api-v3/config/listener/v3/listener.proto.html#config-listener-v3-listener-connectionbalanceconfig-exactbalance

Without this configuration, connections are randomly allocated to Envoy worker threads. Because connections never change threads in Envoy after creation, this can result in uneven distribution of workload. For long-lived, high-traffic connections, it is best to specify this option to guarantee that connection counts are maintained evenly between Envoy workers.

Testing & Reproduction steps

Unit tests were added. Manual testing was performed by using the following configuration and toggling the various permutations of the new fields, then inspecting the Envoy admin config_dump endpoint.

Kind = "service-defaults"
Name = "middle"

BalanceInboundConnections = "exact_balance"
UpstreamConfig {
  Defaults {
    BalanceOutboundConnections = "exact_balance"
  }
  Overrides = [
    {
      Name = "end"
      BalanceOutboundConnections = "exact_balance"
    }
  ]
}

service {
  name = "middle"
  port = 9001

  connect {
    sidecar_service {
      proxy {
        config {
          balance_inbound_connections = "exact_balance"
        }
        upstreams = [
          {
            destination_name = "end"
            local_bind_port  = 5000
            config {
              balance_outbound_connections = "exact_balance"
            }
          },
          {
            destination_name = "something"
            local_bind_port  = 5001
          }
        ]
      }
    }
  }
}

PR Checklist

• updated test coverage
• external facing docs updated
• not a security concern

[skpratt]

Lgtm!

[erichaberkorn]

Looks good 👍

[trujillo-adam]

Made a few suggestions on the docs.