network: add BitRates field to link_build_json() and the Varlink IDL

Expose the speed meter transmit/receive rates as a BitRates struct in
the per-link JSON output and the io.systemd.Network Varlink IDL.

Co-developed-by: Claude Opus 4.6 <noreply@anthropic.com>

src/network/networkd-json.c

@@ -26,6 +26,7 @@
 #include "networkd-route.h"
 #include "networkd-route-util.h"
 #include "networkd-routing-policy-rule.h"
+#include "networkd-speed-meter.h"
 #include "networkd-wwan.h"
 #include "ordered-set.h"
 #include "set.h"
@@ -1639,6 +1640,20 @@ int link_build_json(Link *link, sd_json_variant **ret) {
         if (r < 0)
                 return r;
 
+        /* Append BitRates if speed meter is active */
+        uint64_t tx, rx;
+        link_get_bit_rates(link, &tx, &rx);
+        if (tx != UINT64_MAX && rx != UINT64_MAX) {
+                r = sd_json_variant_merge_objectbo(
+                                &v,
+                                SD_JSON_BUILD_PAIR("BitRates",
+                                        SD_JSON_BUILD_OBJECT(
+                                                SD_JSON_BUILD_PAIR_UNSIGNED("TxBitRate", tx),
+                                                SD_JSON_BUILD_PAIR_UNSIGNED("RxBitRate", rx))));
+                if (r < 0)
+                        return r;
+        }
+
         *ret = TAKE_PTR(v);
         return 0;
 }