add per-task CPU utilisation

Cargo.toml

@@ -104,6 +104,7 @@ instability = "=0.3.10"
 
 # Only for the TUI:
 anyhow = "1.0.104"
+statistical = "1.0.0"
 
 [dev-dependencies]
 tempfile = "=3.14.0"

README.md

@@ -145,7 +145,7 @@ occupies one line.
 
 ```
 INFO vm=vm-a thr=4 iops=155593/0/0 bw_mb_s=20394/0 \
-  cpu=90/358 iops_1_5_15m=154995/153840/151220 \
+  cpu=90/87/358 iops_1_5_15m=154995/153840/151220 \
   cpu_us_per_io_1_5_15m=23/16/19
 ```
 
@@ -154,7 +154,7 @@ INFO vm=vm-a thr=4 iops=155593/0/0 bw_mb_s=20394/0 \
 - `iops`: current read / write / other operations per second.
 - `iops_1_5_15m`: average total IOPS over rolling 1m / 5m / 15m windows.
 - `bw_mb_s`: current read / write bandwidth in MB/s.
-- `cpu`: average per-thread / total pool CPU percentage.
+- `cpu`: average / median / total worker CPU percentage.
 - `cpu_us_per_io_1_5_15m`: CPU microseconds per completed I/O over the same
   windows.
 
@@ -205,6 +205,36 @@ instance's thread count. Setting `sticky=true` suppresses automatic scaling for
 that instance until a later call clears it. This debug-only override is kept in
 memory and is lost when the daemon restarts.
 
+`GetStats()` returns a JSON fleet snapshot. For example:
+
+```json
+{
+  "tick": 42,
+  "vms": [{
+    "vm": "vm-1",
+    "thread_count": 3,
+    "manual_scaling_sticky": false,
+    "scaling_allowed": true,
+    "vcpu_count": 8,
+    "per_thread_util": 0.72,
+    "read_io_count": 1000,
+    "write_io_count": 250,
+    "other_io_count": 0
+  }]
+}
+```
+
+```sh
+busctl --system call \
+  com.nutanix.io_thread_controller1 \
+  /com/nutanix/io_thread_controller1 \
+  com.nutanix.io_thread_controller1 \
+  GetStats
+```
+
+`GetStats` is available in release builds and world-readable under the shipped
+D-Bus policy.
+
 ```sh
 busctl --system call \
   com.nutanix.io_thread_controller1 \