all: release v0.15.0

This commit will be tagged v0.15.0.

Change-Id: Ibc645ddda4af64e7220c10b7343b38f838486a40
Reviewed-on: https://code-review.googlesource.com/c/google-api-go-client/+/49635
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Cody Oss <codyoss@google.com>

Author: jeanbza

CHANGES.md

@@ -1,3 +1,13 @@
+# v0.15.0
+
+- monitoring/v3:
+  - Rename Service to MService; revert APIService to Service.
+
+- Various updates to autogenerated clients.
+
+_Please note:_ the release version is not indicative of an individual client's
+stability or version.
+
 # v0.14.0
 
 - Fix for setting custom HTTP headers in the absence of UserAgent.