Add ftrace support to DebugService

This allows us to do ad-hoc kernel-level tracing on a running Metropolis node.
Useful for tracking down complex bugs.

Example: `bazel run //metropolis/cli/dbg -- trace -function_graph_filter blkdev_* function_graph`

Test Plan: Debug utility, manually tested

X-Origin-Diff: phab/D748
GitOrigin-RevId: 924eb795250412a73eb30c0eef4a8c1cc726e5fd
diff --git a/metropolis/node/core/main.go b/metropolis/node/core/main.go
index 4b19b7f..442102f 100644
--- a/metropolis/node/core/main.go
+++ b/metropolis/node/core/main.go
@@ -208,6 +208,7 @@
 			cluster:    m,
 			logtree:    lt,
 			kubernetes: kubeSvc,
+			traceLock:  make(chan struct{}, 1),
 		}
 		dbgSrv := grpc.NewServer()
 		apb.RegisterNodeDebugServiceServer(dbgSrv, dbg)