smalltown -> metropolis

This pass removes all mentions of Smalltown, both from code and comments,
and replaces them with appropriate new terminology.

Test Plan: Refactor, covered by CI.

X-Origin-Diff: phab/D674
GitOrigin-RevId: 04a94d44ef07d46f7821530da5614daefe16d7ea
diff --git a/metropolis/node/core/debug_service.go b/metropolis/node/core/debug_service.go
index 0155cc6..8e81d2d 100644
--- a/metropolis/node/core/debug_service.go
+++ b/metropolis/node/core/debug_service.go
@@ -37,7 +37,7 @@
 	logFilterMax = 1000
 )
 
-// debugService implements the Smalltown node debug API.
+// debugService implements the Metropolis node debug API.
 type debugService struct {
 	cluster    *cluster.Manager
 	kubernetes *kubernetes.Service