admin: Add optional /debug/pprof/profile endpoint (#2516)

This change introduces a new, optional /debug/pprof/profile.pb.gz
endpoint to the proxy's admin server. This endpoint is feature-gated
and, initially, it will not be included in release builds. It replicates
Go's /debug/pprof/profile, but it always returns gzipped protobuf (i.e.
as can be read by pprof pprofutils).

When the feature is enabled, profiling requests are only permitted over
the loopback interface.

The development Dockerfile is updated to prevent stripping debug
symbols when pprof is enabled so that the pprof data has useful names.

Co-authored-by: Eliza Weisman <[email protected]>

.github/workflows/check-all.yml

@@ -28,3 +28,4 @@ jobs:
       - run: git config --global --add safe.directory "$PWD" # actions/runner#2033
       - run: just fetch
       - run: just check --exclude=linkerd-meshtls-boring
+      - run: just check --exclude=linkerd-meshtls-boring --features=pprof

.vscode/settings.json

@@ -1,4 +1,6 @@
 {
-    "rust-analyzer.cargo.features": [],
+    "rust-analyzer.cargo.features": [
+        "pprof"
+    ],
     "files.insertFinalNewline": true
-}
+}