rxidle and txidle were stats emitted as unsigned 64, now signed (#3519)

CHANGELOG.md

@@ -39,6 +39,7 @@ librdkafka v1.8.0 is a security release:
    is set to 0 - which disables ticket refreshes (by @mpekalski, #3431).
  * Rename internal crc32c() symbol to rd_crc32c() to avoid conflict with
    other static libraries (#3421).
+ * `txidle` and `rxidle` in the statistics object was emitted as 18446744073709551615 when no idle was known. -1 is now emitted instead. (#3519)
 
 
 ### Consumer fixes

src/rdkafka.c

@@ -1703,14 +1703,14 @@ static void rd_kafka_stats_emit_all (rd_kafka_t *rk) {
 			   "\"txbytes\":%"PRIu64", "
 			   "\"txerrs\":%"PRIu64", "
 			   "\"txretries\":%"PRIu64", "
-                           "\"txidle\":%"PRIu64", "
+                           "\"txidle\":%"PRId64", "
 			   "\"req_timeouts\":%"PRIu64", "
 			   "\"rx\":%"PRIu64", "
 			   "\"rxbytes\":%"PRIu64", "
 			   "\"rxerrs\":%"PRIu64", "
                            "\"rxcorriderrs\":%"PRIu64", "
                            "\"rxpartial\":%"PRIu64", "
-                           "\"rxidle\":%"PRIu64", "
+                           "\"rxidle\":%"PRId64", "
                            "\"zbuf_grow\":%"PRIu64", "
                            "\"buf_grow\":%"PRIu64", "
                            "\"wakeups\":%"PRIu64", "