Alerting: Support passing tags to Pagerduty and allow notification on specific event categories (#21335)

Add support for passing tags to Pagerduty and allow notification 
on specific event categories such as Class, Group and Component.

Ref #19912, #19913

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

vendor/github.com/google/go-cmp/cmp/internal/flags/flags.go

@@ -0,0 +1,9 @@
+// Copyright 2019, The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE.md file.
+
+package flags
+
+// Deterministic controls whether the output of Diff should be deterministic.
+// This is only used for testing.
+var Deterministic bool