Add an usage key to the CPU section of OsStats.toXContent.

 This is just the sum of existing `sys` and `user`

Closes elastic#4374

src/main/java/org/elasticsearch/monitor/os/OsStats.java

@@ -116,6 +116,7 @@ static final class Fields {
         static final XContentBuilderString CPU = new XContentBuilderString("cpu");
         static final XContentBuilderString SYS = new XContentBuilderString("sys");
         static final XContentBuilderString USER = new XContentBuilderString("user");
+        static final XContentBuilderString USAGE = new XContentBuilderString("usage");
         static final XContentBuilderString IDLE = new XContentBuilderString("idle");
         static final XContentBuilderString STOLEN = new XContentBuilderString("stolen");
 
@@ -165,6 +166,7 @@ public XContentBuilder toXContent(XContentBuilder builder, Params params) throws
             builder.field(Fields.SYS, cpu.sys());
             builder.field(Fields.USER, cpu.user());
             builder.field(Fields.IDLE, cpu.idle());
+            builder.field(Fields.USAGE, cpu.user() + cpu.sys());
             builder.field(Fields.STOLEN, cpu.stolen());
             builder.endObject();
         }