ops: Add prometheus metrics

Adds a /metrics endpoint in serve mode. Can be disabled via --disable-metrics.

In fetch, dumps the metrics if --metrics is specified (defaults to false).

None of the metrics being collected are on a hot path, so they're just always
collected using atomic operations (i.e. no `if (!enabled) return;`). Because
the operations are cheap and infrequent enough not to matter.

The one place I want to add more metrics is in HttpClient (bytes, status,
counts, ...) but after the pending HttpClient-related PR is merged.

./lightpanda fetch --log-level fatal --metrics "https://lightpanda.io/"

```
build_info{version="1.0.0-dev.7837+70493ce35"} 1
cdp_connections_total 0
cdp_connection_limit_total 0
cdp_active_connections 0
cdp_commands_total 0
cdp_unknown_commands_total 0
js_heap_limits_total 0
script_errors_total 0
arena_hit_total{size="tiny"} 908
arena_hit_total{size="small"} 35
arena_hit_total{size="medium"} 0
arena_hit_total{size="large"} 27
arena_miss_total{size="tiny"} 193
arena_miss_total{size="small"} 31
arena_miss_total{size="medium"} 5
arena_miss_total{size="large"} 23
navigate_total{type="page"} 1
navigate_total{type="iframe"} 0
navigate_total{type="popup"} 0
js_heap_size_bytes_bucket{le="4194304"} 0
js_heap_size_bytes_bucket{le="8388608"} 0
js_heap_size_bytes_bucket{le="16777216"} 1
js_heap_size_bytes_bucket{le="33554432"} 1
js_heap_size_bytes_bucket{le="67108864"} 1
js_heap_size_bytes_bucket{le="134217728"} 1
js_heap_size_bytes_bucket{le="268435456"} 1
js_heap_size_bytes_bucket{le="536870912"} 1
js_heap_size_bytes_bucket{le="+Inf"} 1
js_heap_size_bytes_sum 11223040
js_heap_size_bytes_count 1
```

src/telemetry/telemetry.zig

@@ -109,11 +109,11 @@ pub const Event = union(enum) {
     buffer_overflow: BufferOverflow,
     llm: LLM,
 
-    const Navigate = struct {
+    pub const Navigate = struct {
         tls: bool,
         context: Context,
 
-        const Context = enum { page, iframe, popup };
+        pub const Context = enum { page, iframe, popup };
     };
 
     const BufferOverflow = struct {