http/health: add health checker (#830)

* add health checker

examples/http/health/main.go

@@ -0,0 +1,34 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"log"
+
+	"github.com/go-kratos/kratos/v2"
+	"github.com/go-kratos/kratos/v2/transport/http"
+	"github.com/go-kratos/kratos/v2/transport/http/health"
+)
+
+func main() {
+	handler := health.NewHandler()
+	handler.AddChecker("mysql", func(ctx context.Context) error {
+		return nil
+	})
+	handler.AddObserver("redis", func(ctx context.Context) error {
+		return errors.New("connection refused")
+	})
+
+	httpSrv := http.NewServer(http.Address(":8000"))
+	httpSrv.Handle("/healthz", handler)
+
+	app := kratos.New(
+		kratos.Name("mux"),
+		kratos.Server(
+			httpSrv,
+		),
+	)
+	if err := app.Run(); err != nil {
+		log.Println(err)
+	}
+}