Auto-detect google-project if provider is Google and no project is su… (#492)

* Auto-detect google-project if provider is Google and no project is supplied.

* Add changelog entry.

CHANGELOG.md

@@ -1,3 +1,4 @@
+  - Google Provider: Add auto-detection of Google Project when running on GCP (#492) @drzero42
   - Add custom TTL support for DNSimple (#477) @jbowes
   - Fix docker build and delete vendor files which were not deleted (#473) @njuettner
   - DigitalOcean: DigitalOcean creates entries with host in them twice (#459) @njuettner

provider/google.go

@@ -21,6 +21,7 @@ import (
 	"sort"
 	"strings"
 
+	"cloud.google.com/go/compute/metadata"
 	"github.com/linki/instrumented_http"
 	log "github.com/sirupsen/logrus"
 
@@ -133,6 +134,14 @@ func NewGoogleProvider(project string, domainFilter DomainFilter, zoneIDFilter Z
 		return nil, err
 	}
 
+	if project == "" {
+		mProject, mErr := metadata.ProjectID()
+		if mErr == nil {
+			log.Infof("Google project auto-detected: %s", mProject)
+			project = mProject
+		}
+	}
+
 	provider := &GoogleProvider{
 		project:      project,
 		domainFilter: domainFilter,