You could chose the guage widget, then choose Tier/Node Health - Hardware, JVM, CLR category. Then choose the tier that contains the machine agents that you want to count. Then for the metric choose Agent|Machine|Availability and hit Save. Then for the Value Range, set the max number of machine agents that you have.
If you just want the metric to show up without the guage, you'll have to use a Metric Expression. It's pretty simple: Create a new Metric Widget, and under Metric to display, choose the tier like above. Then instead of a Single Metric, choose Metric Expression, then Edit Expression.
Click Add Variable. For the metric pick agent|machine|availability, then give it a name of activeAgents. Now under the expression you can calculate the percentage. For example, if there are a total of 7 machine agents that can be active in the tier, you would put:
({activeAgents} / 7) * 100
Now you have a percentage of active machine agents. Hit Save, then under Format String put ${v} % and it will display the value with a percentage at the end of it.
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