Merge tag 'v1.26.0' into master

This switches us from the old github.com/golang/protobuf v1 API to the
google.golang.org/protobuf v2 API.

Change-Id: Ia549cb15dde6f5cdcab0d5ed232a16d575513489
diff --git a/METADATA b/METADATA
new file mode 100644
index 0000000..f7a3baf
--- /dev/null
+++ b/METADATA
@@ -0,0 +1,18 @@
+name: "golang-protobuf"
+third_party {
+  url {
+    type: HOMEPAGE
+    value: "https://github.com/protocolbuffers/protobuf"
+  }
+  url {
+    type: GIT
+    value: "https://github.com/protocolbuffers/protobuf"
+  }
+  license_type: NOTICE
+  version: "v1.26.0"
+  last_upgrade_date {
+    year: 2021
+    month: 5
+    day: 24
+  }
+}