config: support weighted URLs in To field (#5624)

## Summary
Currently in core we support weighted URLs like this in the config file:

```yaml
to: 
  - https://a.example.com,1
  - https://b.example.com,2
```

However in the protobuf we use a separate `load_balancing_weights`
field:

```proto
message Route {
  repeated string to                     = 3;
  repeated uint32 load_balancing_weights = 37;
}
```

This PR updates the code to convert from protobuf so that it also
supports weights directly in the `to` addresses. The existing
`load_balancing_weights` behavior is preserved and will take precedence
when provided.

## Related issues
-
[ENG-2398](https://linear.app/pomerium/issue/ENG-2398/enterprise-api-upstream-weight-is-not-parsed)

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

config/policy.go

@@ -4,6 +4,7 @@ import (
 	"crypto/tls"
 	"encoding/base64"
 	"encoding/hex"
+	"errors"
 	"fmt"
 	"net/http"
 	"net/url"
@@ -415,19 +416,16 @@ func NewPolicyFromProto(pb *configpb.Route) (*Policy, error) {
 			Body:   pb.Response.GetBody(),
 		}
 	} else {
-		p.To = make(WeightedURLs, len(pb.To))
-		for i, u := range pb.To {
-			u, err := urlutil.ParseAndValidateURL(u)
-			if err != nil {
-				return nil, err
+		var err error
+		p.To, err = ParseWeightedUrls(pb.To...)
+		if err != nil && !errors.Is(err, errEmptyUrls) {
+			return nil, fmt.Errorf("error parsing to URLs: %w", err)
+		}
+
+		if len(pb.LoadBalancingWeights) == len(p.To) {
+			for i, w := range pb.LoadBalancingWeights {
+				p.To[i].LbWeight = w
 			}
-			w := WeightedURL{
-				URL: *u,
-			}
-			if len(pb.LoadBalancingWeights) == len(pb.To) {
-				w.LbWeight = pb.LoadBalancingWeights[i]
-			}
-			p.To[i] = w
 		}
 	}
 

config/policy_test.go

@@ -16,6 +16,7 @@ import (
 
 	"github.com/pomerium/pomerium/internal/urlutil"
 	"github.com/pomerium/pomerium/pkg/cryptutil"
+	"github.com/pomerium/pomerium/pkg/grpc/config"
 )
 
 func Test_PolicyValidate(t *testing.T) {
@@ -213,6 +214,23 @@ func TestPolicy_Checksum(t *testing.T) {
 	}
 }
 
+func TestNewPolicyFromProto(t *testing.T) {
+	t.Parallel()
+
+	p, err := NewPolicyFromProto(&config.Route{
+		To: []string{"http://127.0.0.1:1234,1", "http://127.0.0.1:1234,2"},
+	})
+	assert.NoError(t, err)
+	assert.Equal(t, mustParseWeightedURLs(t, "http://127.0.0.1:1234,1", "http://127.0.0.1:1234,2"), p.To)
+
+	p, err = NewPolicyFromProto(&config.Route{
+		To:                   []string{"http://127.0.0.1:1234,1", "http://127.0.0.1:1234,2"},
+		LoadBalancingWeights: []uint32{3, 4},
+	})
+	assert.NoError(t, err)
+	assert.Equal(t, mustParseWeightedURLs(t, "http://127.0.0.1:1234,3", "http://127.0.0.1:1234,4"), p.To)
+}
+
 func TestPolicy_FromToPb(t *testing.T) {
 	t.Parallel()
 
@@ -442,7 +460,7 @@ func TestPolicy_IsTCPUpstream(t *testing.T) {
 	assert.False(t, p3.IsTCPUpstream())
 }
 
-func mustParseWeightedURLs(t testing.TB, urls ...string) []WeightedURL {
+func mustParseWeightedURLs(t testing.TB, urls ...string) WeightedURLs {
 	wu, err := ParseWeightedUrls(urls...)
 	require.NoError(t, err)
 	return wu