add version selector wrapper

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

README.md

@@ -0,0 +1,16 @@
+# Version Wrapper
+
+The version wrapper is a stateful client wrapper which gives you a ability to select only latest version services. That suitable for easy upgrade running services without downtime.
+
+## Usage
+
+Pass in the wrapper when you create your service
+
+```
+wrapper := version.NewClientWrapper()
+
+service := micro.NewService(
+	micro.Name("foo"),
+	micro.WrapClient(wrapper),
+)
+```

version.go

@@ -0,0 +1,51 @@
+package version
+
+import (
+	"context"
+	"sort"
+
+	"github.com/micro/go-micro/client"
+	"github.com/micro/go-micro/client/selector"
+	"github.com/micro/go-micro/registry"
+)
+
+// NewClientWrapper is a wrapper which selects only latest versions of services
+func NewClientWrapper() client.Wrapper {
+	return func(c client.Client) client.Client {
+		return &latestVersionWrapper{
+			Client: c,
+		}
+	}
+}
+
+type latestVersionWrapper struct {
+	client.Client
+}
+
+func (w *latestVersionWrapper) Call(ctx context.Context, req client.Request, rsp interface{}, opts ...client.CallOption) error {
+	nOpts := append(opts, client.WithSelectOption(selector.WithFilter(filterLatestVersion())))
+	return w.Client.Call(ctx, req, rsp, nOpts...)
+}
+
+func filterLatestVersion() selector.Filter {
+	return func(svcsOld []*registry.Service) []*registry.Service {
+		var svcsNew []*registry.Service
+		versions := make([]string, len(svcsOld))
+
+		for i, svc := range svcsOld {
+			versions[i] = svc.Version
+		}
+
+		sort.Strings(versions)
+
+		gtVersion := versions[len(versions)-1]
+
+		for _, svc := range svcsOld {
+			if svc.Version == gtVersion {
+				svcsNew = append(svcsNew, svc)
+			}
+		}
+
+		return svcsNew
+	}
+}