[docdb] Initialize timestamp_ms_ for MillisLag (#5081, #5762)

Summary:
Before we were not initializing `timestamp_ms_` which caused flaky errors for `MetricsTest.SimpleLagTest` - changing this to now initialize to `now`.
Also fixing ASAN issue in `MetricsTest::DoLagTest`

Test Plan:
```
ybd --cxx-test util_metrics-test --gtest_filter MetricsTest.SimpleLagTest -n 100
ybd --cxx-test util_metrics-test --gtest_filter MetricsTest.SimpleAtomicLagTest -n 100
```

Reviewers: hector

Reviewed By: hector

Subscribers: bogdan, ybase

Differential Revision: https://phabricator.dev.yugabyte.com/D9431

src/yb/util/metrics-test.cc

@@ -68,16 +68,22 @@ class MetricsTest : public YBTest {
  protected:
   template <class LagType>
   void DoLagTest(const MillisLagPrototype& metric) {
-    auto lag = new LagType(&metric);
+    scoped_refptr<LagType> lag = new LagType(&metric);
     ASSERT_EQ(metric.description(), lag->prototype()->description());
+    SleepFor(MonoDelta::FromMilliseconds(500));
+    // Internal timestamp is set to the time when the metric was created.
+    // So this lag is measure of the time elapsed since the metric was
+    // created and the check time.
+    ASSERT_GE(lag->lag_ms(), 500);
     auto now_ms = std::chrono::duration_cast<std::chrono::milliseconds>(
         std::chrono::system_clock::now().time_since_epoch()).count();
-    SleepFor(MonoDelta::FromMilliseconds(100));
-    ASSERT_LT(now_ms, lag->lag_ms());
     lag->UpdateTimestampInMilliseconds(now_ms);
-    ASSERT_GT(10000, lag->lag_ms());
-    // Set the timestamp to some time in the future to verify that the metric can correctly deal
-    // with this case.
+    // Verify that the update happened correctly. The lag time should
+    // be close to 0, but giving it extra time to account for slow
+    // tests.
+    ASSERT_LT(lag->lag_ms(), 200);
+    // Set the timestamp to some time in the future to verify that the
+    // metric can correctly deal with this case.
     lag->UpdateTimestampInMilliseconds(now_ms * 2);
     ASSERT_EQ(0, lag->lag_ms());
   }

src/yb/util/metrics.cc

@@ -780,7 +780,10 @@ scoped_refptr<MillisLag> MillisLagPrototype::Instantiate(
   return entity->FindOrCreateMillisLag(this);
 }
 
-MillisLag::MillisLag(const MillisLagPrototype* proto) : Metric(proto) {
+MillisLag::MillisLag(const MillisLagPrototype* proto)
+  : Metric(proto),
+    timestamp_ms_(static_cast<int64_t>(std::chrono::duration_cast<std::chrono::milliseconds>(
+        std::chrono::system_clock::now().time_since_epoch()).count())) {
 }
 
 Status MillisLag::WriteAsJson(JsonWriter* writer, const MetricJsonOptions& opts) const {
@@ -809,6 +812,12 @@ Status MillisLag::WriteForPrometheus(
   return writer->WriteSingleEntry(attr, prototype_->name(), lag_ms());
 }
 
+AtomicMillisLag::AtomicMillisLag(const MillisLagPrototype* proto)
+  : MillisLag(proto),
+    atomic_timestamp_ms_(static_cast<int64_t>(std::chrono::duration_cast<std::chrono::milliseconds>(
+        std::chrono::system_clock::now().time_since_epoch()).count())) {
+}
+
 Status AtomicMillisLag::WriteAsJson(JsonWriter* writer, const MetricJsonOptions& opts) const {
   if (prototype_->level() < opts.level) {
     return Status::OK();

src/yb/util/metrics.h

@@ -1345,7 +1345,7 @@ class MillisLag : public Metric {
 
 class AtomicMillisLag : public MillisLag {
  public:
-  explicit AtomicMillisLag(const MillisLagPrototype* proto) : MillisLag(proto) {}
+  explicit AtomicMillisLag(const MillisLagPrototype* proto);
 
   int64_t lag_ms() const override {
     return std::max(static_cast<int64_t>(0),