otlpmetricgrpc: error setting OTEL_EXPORTER_OTLP_METRICS_ENDPOINT with a path

[RedZapdos123]

Description:

Setting OTEL_EXPORTER_OTLP_METRICS_ENDPOINT to a URL with a path causes the OTLP metrics gRPC exporter to build an invalid gRPC target by joining the host and path.

On current main, withEndpointForGRPC in exporters/otlp/otlpmetric/otlpmetricgrpc/internal/oconf/envconfig.go sets:

cfg.Metrics.Endpoint = path.Join(u.Host, u.Path)

So an endpoint like http://127.0.0.1:1/v1/metrics becomes the gRPC dial target 127.0.0.1:1/v1/metrics.

This looks like the metrics-side sibling of #7831.

Environment:

• OS: Linux 6.6.87.2-microsoft-standard-WSL2.
• Architecture: x86_64.
• Go Version: go1.26.4 linux/amd64.
• opentelemetry-go version: main at 524fc3c25.

Steps To Reproduce:

1. Check out main at 524fc3c25.
2. From the repository root, run.

cd exporters/otlp/otlpmetric/otlpmetricgrpc
mkdir -p tmprepro
cat > tmprepro/main.go <<'EOF'
package main

import (
	"context"
	"errors"
	"fmt"
	"net"
	"time"

	otlpmetricgrpc "go.opentelemetry.io/otel/exporters/otlp/otlpmetric/otlpmetricgrpc"
	sdkmetric "go.opentelemetry.io/otel/sdk/metric"
	"google.golang.org/grpc"
)

func main() {
	ctx := context.Background()
	exp, err := otlpmetricgrpc.New(
		ctx,
		otlpmetricgrpc.WithRetry(otlpmetricgrpc.RetryConfig{Enabled: false}),
		otlpmetricgrpc.WithDialOption(grpc.WithContextDialer(func(ctx context.Context, target string) (net.Conn, error) {
			return nil, errors.New(target)
		})),
	)
	if err != nil {
		panic(err)
	}

	mp := sdkmetric.NewMeterProvider(
		sdkmetric.WithReader(sdkmetric.NewPeriodicReader(exp, sdkmetric.WithInterval(time.Hour))),
	)

	c, err := mp.Meter("repro").Int64Counter("c")
	if err != nil {
		panic(err)
	}
	c.Add(ctx, 1)

	err = mp.ForceFlush(context.Background())
	fmt.Println(err)
}
EOF

OTEL_EXPORTER_OTLP_METRICS_ENDPOINT=http://127.0.0.1:1/v1/metrics go run ./tmprepro
rm -rf tmprepro

Actual behavior:

The exporter tries to dial a target that still contains the path:

failed to upload metrics: rpc error: code = Unavailable desc = connection error: desc = "transport: Error while dialing: 127.0.0.1:1/v1/metrics"

Expected behavior:

The gRPC dial target should not include the URL path.

At minimum, OTEL_EXPORTER_OTLP_METRICS_ENDPOINT=http://127.0.0.1:1/v1/metrics should not produce a target like 127.0.0.1:1/v1/metrics. The exporter should either:

• use only 127.0.0.1:1 as the gRPC endpoint target.
• reject endpoint URLs with a path clearly during configuration.

Notes:

This appears to be the same root problem as #7831, but in otlpmetricgrpc instead of otlptracegrpc.

[RedZapdos123]

Opened a PR to resolve the issue.