fix(hive-metric): multi-thread runtime + counter default

docs/observability.md

@@ -136,13 +136,13 @@ Agents can push arbitrary labeled metrics to the same OTEL collector via the
 ### Usage
 
 ```text
-hive-metric <name> <value> [--type gauge|counter] [--labels key=value...]
+hive-metric <name> <value> [--type counter|gauge] [--labels key=value...]
 ```
 
 - `<name>` — metric name (e.g. `tasks_completed`, `latency_ms`).
 - `<value>` — numeric value (f64; integers and floats both accepted).
-- `--type gauge|counter` — metric kind: `gauge` (instantaneous, default) or
-  `counter` (monotonically increasing cumulative sum).
+- `--type counter|gauge` — metric kind: `counter` (cumulative sum, default) or
+  `gauge` (instantaneous point-in-time value).
 - `--labels key=value` — extra per-data-point labels. May be repeated.
   The resource labels (agent, hive, swarm, service.name) are inherited
   automatically from `OTEL_RESOURCE_ATTRIBUTES` — do not re-specify them.
@@ -150,14 +150,14 @@ hive-metric <name> <value> [--type gauge|counter] [--labels key=value...]
 ### Examples
 
 ```text
-# Gauge: current queue depth
-hive-metric queue_depth 17
+# Counter: cumulative tasks finished (default type — no --type flag needed)
+hive-metric tasks_completed 1 --labels phase=scan
 
-# Counter: cumulative tasks finished, with a custom label
-hive-metric tasks_completed 1 --type counter --labels phase=scan
+# Gauge: current queue depth (absolute value — must use --type gauge)
+hive-metric queue_depth 17 --type gauge
 
-# Float gauge with multiple labels
-hive-metric api_latency_ms 142.5 --labels model=sonnet --labels tier=api
+# Float gauge with multiple labels (instantaneous measurement)
+hive-metric api_latency_ms 142.5 --type gauge --labels model=sonnet --labels tier=api
 ```
 
 ### Error when OTEL is not configured