config: add circuit breaker thresholds (#5650)

## Summary
Add a new `circuit_breaker_thresholds` option:

```yaml
circuit_breaker_thresholds:
  max_connections: 1
  max_pending_requests: 2
  max_requests: 3
  max_retries: 4
  max_connection_pools: 5
```

This option can be set at the global level or at the route level. Each
threshold is optional and when not set a default will be used. For
internal clusters we will disable the circuit breaker. For normal routes
we will use the envoy defaults.

## Related issues
-
[ENG-2310](https://linear.app/pomerium/issue/ENG-2310/add-circuit-breaker-settings-per-route)

## Checklist
- [x] reference any related issues
- [x] updated unit tests
- [x] add appropriate label (`enhancement`, `bug`, `breaking`,
`dependencies`, `ci`)
- [x] ready for review

config/envoyconfig/circuit_breakers.go

@@ -0,0 +1,83 @@
+package envoyconfig
+
+import (
+	"math"
+
+	envoy_config_cluster_v3 "github.com/envoyproxy/go-control-plane/envoy/config/cluster/v3"
+	envoy_config_core_v3 "github.com/envoyproxy/go-control-plane/envoy/config/core/v3"
+	"google.golang.org/protobuf/proto"
+	"google.golang.org/protobuf/types/known/wrapperspb"
+
+	"github.com/pomerium/pomerium/config"
+)
+
+// unlimitedCircuitBreakersThreshold sets the circuit breaking thresholds to the maximum value, effectively disabling them
+var unlimitedCircuitBreakersThreshold = &envoy_config_cluster_v3.CircuitBreakers_Thresholds{
+	Priority:           envoy_config_core_v3.RoutingPriority_DEFAULT,
+	MaxConnections:     wrapperspb.UInt32(math.MaxUint32),
+	MaxPendingRequests: wrapperspb.UInt32(math.MaxUint32),
+	MaxRequests:        wrapperspb.UInt32(math.MaxUint32),
+	MaxConnectionPools: wrapperspb.UInt32(math.MaxUint32),
+}
+
+func buildInternalCircuitBreakers(cfg *config.Config) *envoy_config_cluster_v3.CircuitBreakers {
+	threshold := unlimitedCircuitBreakersThreshold
+	if cfg != nil && cfg.Options != nil {
+		threshold = buildCircuitBreakersThreshold(threshold, cfg.Options.CircuitBreakerThresholds)
+	}
+	if threshold == nil {
+		return nil
+	}
+
+	return &envoy_config_cluster_v3.CircuitBreakers{
+		Thresholds: []*envoy_config_cluster_v3.CircuitBreakers_Thresholds{threshold},
+	}
+}
+
+func buildRouteCircuitBreakers(cfg *config.Config, policy *config.Policy) *envoy_config_cluster_v3.CircuitBreakers {
+	threshold := (*envoy_config_cluster_v3.CircuitBreakers_Thresholds)(nil)
+	if cfg != nil && cfg.Options != nil {
+		threshold = buildCircuitBreakersThreshold(threshold, cfg.Options.CircuitBreakerThresholds)
+	}
+	if policy != nil {
+		threshold = buildCircuitBreakersThreshold(threshold, policy.CircuitBreakerThresholds)
+	}
+
+	if threshold == nil {
+		return nil
+	}
+
+	return &envoy_config_cluster_v3.CircuitBreakers{
+		Thresholds: []*envoy_config_cluster_v3.CircuitBreakers_Thresholds{threshold},
+	}
+}
+
+func buildCircuitBreakersThreshold(dst *envoy_config_cluster_v3.CircuitBreakers_Thresholds, src *config.CircuitBreakerThresholds) *envoy_config_cluster_v3.CircuitBreakers_Thresholds {
+	if src == nil {
+		return dst
+	}
+
+	if dst == nil {
+		dst = new(envoy_config_cluster_v3.CircuitBreakers_Thresholds)
+	} else {
+		dst = proto.CloneOf(dst)
+	}
+
+	if src.MaxConnections.IsSet() {
+		dst.MaxConnections = wrapperspb.UInt32(src.MaxConnections.Uint32)
+	}
+	if src.MaxPendingRequests.IsSet() {
+		dst.MaxPendingRequests = wrapperspb.UInt32(src.MaxPendingRequests.Uint32)
+	}
+	if src.MaxRequests.IsSet() {
+		dst.MaxRequests = wrapperspb.UInt32(src.MaxRequests.Uint32)
+	}
+	if src.MaxRetries.IsSet() {
+		dst.MaxRetries = wrapperspb.UInt32(src.MaxRetries.Uint32)
+	}
+	if src.MaxConnectionPools.IsSet() {
+		dst.MaxConnectionPools = wrapperspb.UInt32(src.MaxConnectionPools.Uint32)
+	}
+
+	return dst
+}