[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() to cpuwide_time_ns(), but the surrounding code still
treats the timestamps as microseconds:

- abstime += 1000000L now represents 1 ms (not 1 s), causing the
  sampler to spin at ~1 kHz instead of 1 Hz;
- usleep(abstime - now) receives a nanosecond delta, which usleep()
  interprets as microseconds.

Use cpuwide_time_us() instead, which preserves the monotonic behavior
from #3268 while keeping the existing microsecond-based arithmetic
correct.

Fixes #3277.

Author: huangjun

src/bvar/detail/sampler.cpp

@@ -155,7 +155,7 @@ void SamplerCollector::run() {
     butil::LinkNode<Sampler> root;
     int consecutive_nosleep = 0;
     while (!_stop) {
-        int64_t abstime = butil::cpuwide_time_ns();
+        int64_t abstime = butil::cpuwide_time_us();
         Sampler* s = this->reset();
         if (s) {
             s->InsertBeforeAsList(&root);
@@ -176,13 +176,13 @@ void SamplerCollector::run() {
             p = saved_next;
         }
         bool slept = false;
-        int64_t now = butil::cpuwide_time_ns();
+        int64_t now = butil::cpuwide_time_us();
         _cumulated_time_us += now - abstime;
         abstime += 1000000L;
         while (abstime > now) {
             ::usleep(abstime - now);
             slept = true;
-            now = butil::cpuwide_time_ns();
+            now = butil::cpuwide_time_us();
         }
         if (slept) {
             consecutive_nosleep = 0;