Upgrade to go-execute/v2

Upgrades to go-execute/v2 and updates various other
dependencies.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

vendor/github.com/prometheus/common/expfmt/decode.go

@@ -22,7 +22,7 @@ import (
 
 	dto "github.com/prometheus/client_model/go"
 
-	"github.com/matttproud/golang_protobuf_extensions/pbutil"
+	"github.com/matttproud/golang_protobuf_extensions/v2/pbutil"
 	"github.com/prometheus/common/model"
 )
 
@@ -132,7 +132,10 @@ func (d *textDecoder) Decode(v *dto.MetricFamily) error {
 	}
 	// Pick off one MetricFamily per Decode until there's nothing left.
 	for key, fam := range d.fams {
-		*v = *fam
+		v.Name = fam.Name
+		v.Help = fam.Help
+		v.Type = fam.Type
+		v.Metric = fam.Metric
 		delete(d.fams, key)
 		return nil
 	}