Add generic write path using grpc.

This uses a new proto format, with scope for multiple samples per
timeseries in future. This will allow users to pump samples out to
whatever they like without having to change the core Prometheus code.

There's also an example receiver to save users figuring out the
boilerplate themselves.

documentation/examples/remote_storage_generic/README.md

@@ -0,0 +1,17 @@
+## Generic Remote Storage Example
+
+This is a simple example of how to write a server to
+recieve samples from the generic remote storage output.
+
+To use it:
+
+```
+go build
+remote_storage_generic
+```
+
+and then run Prometheus as:
+
+```
+./prometheus -storage.remote.generic-url http://localhost:1234/remote  
+```

documentation/examples/remote_storage_generic/server.go

@@ -0,0 +1,53 @@
+// Copyright 2016 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"fmt"
+	"log"
+	"net"
+
+	"golang.org/x/net/context"
+	"google.golang.org/grpc"
+
+	"github.com/prometheus/prometheus/storage/remote/generic"
+)
+
+type server struct{}
+
+func (server *server) Write(ctx context.Context, req *generic.GenericWriteRequest) (*generic.GenericWriteResponse, error) {
+	for _, ts := range req.Timeseries {
+		fmt.Printf("%s", ts.Name)
+		for _, l := range ts.Labels {
+			fmt.Printf(" %s=%s", l.Name, l.Value)
+		}
+		fmt.Printf("\n")
+
+		for _, s := range ts.Samples {
+			fmt.Printf("  %f %d\n", s.Value, s.TimestampMs)
+		}
+	}
+
+	return &generic.GenericWriteResponse{}, nil
+}
+
+func main() {
+	lis, err := net.Listen("tcp", ":1234")
+	if err != nil {
+		log.Fatalf("failed to listen: %v", err)
+	}
+	s := grpc.NewServer()
+	generic.RegisterGenericWriteServer(s, &server{})
+	s.Serve(lis)
+}