docs: specify possible values for ProcessMetric.type (#19224)

docs/api/structures/process-metric.md

@@ -1,5 +1,14 @@
 # ProcessMetric Object
 
 * `pid` Integer - Process id of the process.
-* `type` String - Process type (Browser or Tab or GPU etc).
+* `type` String - Process type. One of the following values:
+  * `Browser`
+  * `Tab`
+  * `Utility`
+  * `Zygote`
+  * `Sandbox helper`
+  * `GPU`
+  * `Pepper Plugin`
+  * `Pepper Plugin Broker`
+  * `Unknown`
 * `cpu` [CPUUsage](cpu-usage.md) - CPU usage of the process.