[bvar] fix sampler interval after switch to cpuwide_time_ns

Commit 12fb539 ("Use monotonic time instead of wall time", #3268)
switched the three time-source calls in
SamplerCollector::run() from gettimeofday_us() (microseconds) to
cpuwide_time_ns() (nanoseconds), but left the 1-second offset constant
and the usleep() call untouched. As a result:

- abstime += 1000000L now adds 1 ms instead of 1 s, so the sampler
  spins at ~1 kHz instead of 1 Hz;
- usleep(abstime - now) is passed a nanosecond delta that usleep()
  interprets as microseconds, which further distorts the sleep.

Fix by using a 1-second offset expressed in nanoseconds and by
converting the nanosecond delta to microseconds before handing it to
usleep().

Fixes #3277.

Author: huangjun

src/bvar/detail/sampler.cpp

@@ -178,9 +178,9 @@ void SamplerCollector::run() {
         bool slept = false;
         int64_t now = butil::cpuwide_time_ns();
         _cumulated_time_us += now - abstime;
-        abstime += 1000000L;
+        abstime += 1000000000L;  // 1 second, in nanoseconds
         while (abstime > now) {
-            ::usleep(abstime - now);
+            ::usleep((abstime - now) / 1000);  // usleep() takes microseconds
             slept = true;
             now = butil::cpuwide_time_ns();
         }