docs: Tracing doc

Add `tracing.md` to document how to use the instrumentation to trace the
Firecracker process.

Signed-off-by: Jonathan Woollett-Light <[email protected]>

docs/tracing.md

@@ -0,0 +1,32 @@
+# Tracing
+
+Instrumentation based tracing is implemented with [`log`](https://github.com/rust-lang/log) and [`log_instrument`](https://github.com/JonathanWoollett-Light/log-instrument), outputing a `Trace` level log entering and exiting every function.
+
+It is disabled by default at compile-time to remove any impact it would have on the release builds.
+
+To enable it you need to remove `"max_level_debug"` from the features for `log` under [`../src/vmm/Cargo.toml`](../src/vmm/Cargo.toml):
+
+```diff
+- log = { version = "0.4.17", features = ["std", "serde", "max_level_debug"] }
++ log = { version = "0.4.17", features = ["std", "serde"] }
+```
+
+This will result in a signficant increase in the binary size and a signficant regression in performance.
+
+You can filter by module and file this can be done like:
+
+```bash
+curl -X PUT --unix-socket "${API_SOCKET}" \
+    --data "{
+        \"log_path\": \"${LOGFILE}\",
+        # Instrumentation logs are `Trace` level, at runtime the level must be set to `Trace` to see them.
+        \"level\": \"Trace\",
+        \"show_level\": true,
+        \"show_log_origin\": true,
+        # Only outputs logs from `src/firecracker/src/api_server_adapter.rs`.
+        \"file\": \"src/firecracker/src/api_server_adapter.rs\",
+    }" \
+    "http://localhost/logger"
+```
+
+This can mitigate most of the performance regression.