Reenable the dartdocs benchmark tracking (#42882)

dev/devicelab/manifest.yaml

diff --git a/dev/devicelab/manifest.yaml b/dev/devicelab/manifest.yaml
index dbeef6c..8909b92 100644
--- a/dev/devicelab/manifest.yaml
+++ b/dev/devicelab/manifest.yaml
@@ -607,6 +607,12 @@
     stage: devicelab
     required_agent_capabilities: ["linux/android"]
 
+  dartdocs:
+    description: >
+      Tracks how many members are still lacking documentation.
+    stage: devicelab
+    required_agent_capabilities: ["linux/android"]
+
   technical_debt__cost:
     description: >
       Estimates our technical debt (TODOs, analyzer ignores, etc).