feat: update filter to accept an exact timestamp for better accessibility (#92)

This change introduces `FILTERS.timestamp().exact()`, which enable user to directly provide exact timestamp to query Bigtable.

Author: rahulKQL

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/models/Filters.java

@@ -516,6 +516,15 @@ private TimestampFilter() {}
     public TimestampRangeFilter range() {
       return new TimestampRangeFilter();
     }
+
+    /**
+     * Matches cells with exact given timestamp.
+     *
+     * @return a {@link TimestampRangeFilter} with start/end closed timestamp.
+     */
+    public TimestampRangeFilter exact(Long exactTimestamp) {
+      return new TimestampRangeFilter().startClosed(exactTimestamp).endClosed(exactTimestamp);
+    }
   }
 
   /** Matches only cells with microsecond timestamps within the given range. */

google-cloud-bigtable/src/test/java/com/google/cloud/bigtable/data/v2/models/FiltersTest.java

@@ -350,6 +350,21 @@ public void timestampRange() {
     assertThat(actualFilter).isEqualTo(expectedFilter);
   }
 
+  @Test
+  public void timestampAtExactTime() {
+    RowFilter actualFilter = FILTERS.timestamp().exact(20_000L).toProto();
+
+    RowFilter expectedFilter =
+        RowFilter.newBuilder()
+            .setTimestampRangeFilter(
+                TimestampRange.newBuilder()
+                    .setStartTimestampMicros(20_000L)
+                    .setEndTimestampMicros(20_000L + 1))
+            .build();
+
+    assertThat(actualFilter).isEqualTo(expectedFilter);
+  }
+
   @Test
   public void timestampOpenClosedFakeRange() {
     RowFilter actualFilter =