core/zero: add usage reporter (#5281)

* wip

* add response

* handle empty email

* use set, update log

* add test

* add coalesce, comments, test

* add test, fix bug

* use builtin cmp.Or

* remove wait ready call

* use api error

internal/testutil/grpc.go

@@ -0,0 +1,44 @@
+package testutil
+
+import (
+	"context"
+	"errors"
+	"net"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/credentials/insecure"
+	"google.golang.org/grpc/test/bufconn"
+)
+
+// NewGRPCServer starts a gRPC server and returns a client connection to it.
+func NewGRPCServer(t testing.TB, register func(s *grpc.Server)) *grpc.ClientConn {
+	t.Helper()
+
+	li := bufconn.Listen(1024 * 1024)
+	s := grpc.NewServer()
+	register(s)
+	go func() {
+		err := s.Serve(li)
+		if errors.Is(err, grpc.ErrServerStopped) {
+			err = nil
+		}
+		require.NoError(t, err)
+	}()
+	t.Cleanup(func() {
+		s.Stop()
+	})
+
+	cc, err := grpc.NewClient("passthrough://bufnet",
+		grpc.WithContextDialer(func(context.Context, string) (net.Conn, error) {
+			return li.Dial()
+		}),
+		grpc.WithTransportCredentials(insecure.NewCredentials()))
+	require.NoError(t, err)
+	t.Cleanup(func() {
+		cc.Close()
+	})
+
+	return cc
+}