(otel): onboard ateom to the OTLP metrics path (#562)

This PR wires `ateom` into the OTLP push path so it can emit metrics,
and unblock unblocking
https://github.com/agent-substrate/substrate/issues/550

- serverboot: new InitMetricsPushOnly (OTLP periodic reader, no
Prometheus pull surface) for binaries that run no metrics HTTP server
- ateom-gvisor + ateom-microvm call it, so the existing otelgrpc server
handler now feeds `rpc.server.*` to the collector
- atecontroller propagates its `OTEL_EXPORTER_OTLP_ENDPOINT` +
`OTEL_RESOURCE_ATTRIBUTES` into the ateom worker pods
- fixes ateom telemetry not reaching the collector 
- e2e: metrics suite asserts an `ateom` service reaches the collector

cc. @git286 

- [x] Tests pass
- [ ] Appropriate changes to documentation are included in the PR

internal/serverboot/serverboot.go

@@ -13,9 +13,9 @@
 // limitations under the License.
 
 // Package serverboot collects the startup boilerplate shared by the
-// long-running substrate server binaries (ateapi, atelet, ateom-gvisor):
-// slog wiring, OTel tracer + meter providers, a Prometheus + /readyz
-// HTTP surface, and a couple of small helpers for startup fail-fast.
+// long-running substrate server binaries (ateapi, atelet, ateom-gvisor,
+// ateom-microvm): slog wiring, OTel tracer + meter providers, a Prometheus +
+// /readyz HTTP surface, and a couple of small helpers for startup fail-fast.
 package serverboot
 
 import (
@@ -130,6 +130,20 @@ func InitMetrics(ctx context.Context, serviceName string) (*sdkmetric.MeterProvi
 	if err != nil {
 		return nil, fmt.Errorf("create Prometheus metric exporter: %w", err)
 	}
+	return newMeterProvider(ctx, serviceName, promExporter)
+}
+
+// InitMetricsPushOnly is InitMetrics without the Prometheus reader, for binaries
+// that run no metrics HTTP server (ateom): a pull reader would collect into a
+// registry nothing serves.
+func InitMetricsPushOnly(ctx context.Context, serviceName string) (*sdkmetric.MeterProvider, error) {
+	return newMeterProvider(ctx, serviceName)
+}
+
+func newMeterProvider(ctx context.Context, serviceName string, extraReaders ...sdkmetric.Reader) (*sdkmetric.MeterProvider, error) {
+	if serviceName == "" {
+		return nil, fmt.Errorf("serviceName is required")
+	}
 	otlpExporter, err := otlpmetricgrpc.New(ctx, otlpmetricgrpc.WithInsecure())
 	if err != nil {
 		return nil, fmt.Errorf("create OTLP metric exporter: %w", err)
@@ -138,11 +152,14 @@ func InitMetrics(ctx context.Context, serviceName string) (*sdkmetric.MeterProvi
 	if err != nil {
 		return nil, fmt.Errorf("create metric resource: %w", err)
 	}
-	mp := sdkmetric.NewMeterProvider(
-		sdkmetric.WithReader(promExporter),
-		sdkmetric.WithReader(sdkmetric.NewPeriodicReader(otlpExporter)),
+	opts := []sdkmetric.Option{
 		sdkmetric.WithResource(res),
-	)
+		sdkmetric.WithReader(sdkmetric.NewPeriodicReader(otlpExporter)),
+	}
+	for _, r := range extraReaders {
+		opts = append(opts, sdkmetric.WithReader(r))
+	}
+	mp := sdkmetric.NewMeterProvider(opts...)
 	otel.SetMeterProvider(mp)
 	return mp, nil
 }

internal/serverboot/serverboot_test.go

@@ -18,8 +18,11 @@ import (
 	"context"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
+	"time"
 
+	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"go.opentelemetry.io/otel/sdk/resource"
 	semconv "go.opentelemetry.io/otel/semconv/v1.21.0"
 )
@@ -107,6 +110,46 @@ func TestHealthzAbsentUnlessEnabled(t *testing.T) {
 	}
 }
 
+func TestInitMetricsPushOnlyHasNoPrometheusSurface(t *testing.T) {
+	mp, err := InitMetricsPushOnly(context.Background(), "test-pushonly")
+	if err != nil {
+		t.Fatalf("InitMetricsPushOnly: %v", err)
+	}
+	// Bound shutdown: the periodic reader would otherwise block flushing to the
+	// unreachable default OTLP endpoint until the export timeout.
+	t.Cleanup(func() {
+		ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+		defer cancel()
+		_ = mp.Shutdown(ctx)
+	})
+
+	ctr, err := mp.Meter("test").Int64Counter("ate.test.pushonly.count")
+	if err != nil {
+		t.Fatalf("create counter: %v", err)
+	}
+	ctr.Add(context.Background(), 1)
+
+	// A push-only provider registers no Prometheus reader, so what it records must
+	// not surface on the default registry StartMetricsServer's /metrics serves.
+	rec := httptest.NewRecorder()
+	promhttp.Handler().ServeHTTP(rec, httptest.NewRequest(http.MethodGet, "/metrics", nil))
+	if strings.Contains(rec.Body.String(), "ate_test_pushonly") {
+		t.Error("push-only MeterProvider must not expose a Prometheus pull surface")
+	}
+}
+
+func TestInitMetricsPushOnlyRequiresServiceName(t *testing.T) {
+	if _, err := InitMetricsPushOnly(context.Background(), ""); err == nil {
+		t.Error("InitMetricsPushOnly(\"\") must return an error")
+	}
+}
+
+func TestInitMetricsRequiresServiceName(t *testing.T) {
+	if _, err := InitMetrics(context.Background(), ""); err == nil {
+		t.Error("InitMetrics(\"\") must return an error")
+	}
+}
+
 func getCode(t *testing.T, mux *http.ServeMux, path string) int {
 	t.Helper()
 	rec := httptest.NewRecorder()