Update all dependencies.

* Switch to using kingpin for CLI interface as upstream Prometheus has moved
  that way too.

vendor/github.com/golang/protobuf/proto/lib.go

@@ -70,6 +70,11 @@ for a protocol buffer variable v:
 	with distinguished wrapper types for each possible field value.
   - Marshal and Unmarshal are functions to encode and decode the wire format.
 
+When the .proto file specifies `syntax="proto3"`, there are some differences:
+
+  - Non-repeated fields of non-message type are values instead of pointers.
+  - Enum types do not get an Enum method.
+
 The simplest way to describe this is to see an example.
 Given file test.proto, containing
 
@@ -229,6 +234,7 @@ To create and play with a Test object:
 		test := &pb.Test{
 			Label: proto.String("hello"),
 			Type:  proto.Int32(17),
+			Reps:  []int64{1, 2, 3},
 			Optionalgroup: &pb.Test_OptionalGroup{
 				RequiredField: proto.String("good bye"),
 			},
@@ -301,7 +307,7 @@ func GetStats() Stats { return stats }
 // temporary Buffer and are fine for most applications.
 type Buffer struct {
 	buf   []byte // encode/decode byte stream
-	index int    // write point
+	index int    // read point
 
 	// pools of basic types to amortize allocation.
 	bools   []bool
@@ -881,3 +887,11 @@ func isProto3Zero(v reflect.Value) bool {
 	}
 	return false
 }
+
+// ProtoPackageIsVersion2 is referenced from generated protocol buffer files
+// to assert that that code is compatible with this version of the proto package.
+const ProtoPackageIsVersion2 = true
+
+// ProtoPackageIsVersion1 is referenced from generated protocol buffer files
+// to assert that that code is compatible with this version of the proto package.
+const ProtoPackageIsVersion1 = true