metrics: send metrics to the otel collector endpoint when active

Introduce a meter provider to the buildx cli that will send metrics to
the otel-collector included in docker desktop if enabled.

This will send usage metrics to the desktop application but also send
metrics to a user-provided otlp receiver endpoint through the standard
environment variables.

This introduces a single metric which is the cli count for build and
bake along with the command name and a few additional attributes.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

util/metrics/otlp.go

@@ -0,0 +1,49 @@
+package metrics
+
+import (
+	"context"
+	"os"
+
+	"github.com/pkg/errors"
+	"go.opentelemetry.io/otel/exporters/otlp/otlpmetric/otlpmetricgrpc"
+	"go.opentelemetry.io/otel/exporters/otlp/otlpmetric/otlpmetrichttp"
+	sdkmetric "go.opentelemetry.io/otel/sdk/metric"
+)
+
+// detectOtlpExporter configures a metrics exporter based on environment variables.
+// This is similar to the version of this in buildkit, but we need direct access
+// to the exporter and the prometheus exporter doesn't work at all in a CLI context.
+//
+// There's some duplication here which I hope to remove when the detect package
+// is refactored or extracted from buildkit so it can be utilized here.
+//
+// This version of the exporter is public facing in contrast to the
+// docker otel collector.
+func detectOtlpExporter(ctx context.Context) (sdkmetric.Exporter, error) {
+	set := os.Getenv("OTEL_METRICS_EXPORTER") == "otlp" || os.Getenv("OTEL_EXPORTER_OTLP_ENDPOINT") != "" || os.Getenv("OTEL_EXPORTER_OTLP_METRICS_ENDPOINT") != ""
+	if !set {
+		return nil, nil
+	}
+
+	proto := os.Getenv("OTEL_EXPORTER_OTLP_METRICS_PROTOCOL")
+	if proto == "" {
+		proto = os.Getenv("OTEL_EXPORTER_OTLP_PROTOCOL")
+	}
+	if proto == "" {
+		proto = "grpc"
+	}
+
+	switch proto {
+	case "grpc":
+		return otlpmetricgrpc.New(ctx,
+			otlpmetricgrpc.WithTemporalitySelector(deltaTemporality),
+		)
+	case "http/protobuf":
+		return otlpmetrichttp.New(ctx,
+			otlpmetrichttp.WithTemporalitySelector(deltaTemporality),
+		)
+	// case "http/json": // unsupported by library
+	default:
+		return nil, errors.Errorf("unsupported otlp protocol %v", proto)
+	}
+}