Fix fractional seconds for strict_date_optional_time (#41871)

The fractional seconds portion of strict_date_optional_time was
accidentally copied from the printer, which always prints at least 3
fractional digits. This commit fixes the formatter to allow 1 or 2
fractional seconds.

closes #41633

server/src/main/java/org/elasticsearch/common/time/DateFormatters.java

@@ -110,7 +110,7 @@ public class DateFormatters {
         .appendLiteral(':')
         .appendValue(SECOND_OF_MINUTE, 2, 2, SignStyle.NOT_NEGATIVE)
         .optionalStart()
-        .appendFraction(NANO_OF_SECOND, 3, 9, true)
+        .appendFraction(NANO_OF_SECOND, 1, 9, true)
         .optionalEnd()
         .optionalEnd()
         .optionalStart()

server/src/test/java/org/elasticsearch/common/time/DateFormattersTests.java

@@ -260,4 +260,44 @@ public class DateFormattersTests extends ESTestCase {
         String formatted = formatter.formatMillis(clock.millis());
         assertThat(formatted, is("2019-02-08T11:43:00.000Z"));
     }
+
+    public void testFractionalSeconds() {
+        DateFormatter formatter = DateFormatters.forPattern("strict_date_optional_time");
+        {
+            Instant instant = Instant.from(formatter.parse("2019-05-06T14:52:37.1Z"));
+            assertThat(instant.getNano(), is(100_000_000));
+        }
+        {
+            Instant instant = Instant.from(formatter.parse("2019-05-06T14:52:37.12Z"));
+            assertThat(instant.getNano(), is(120_000_000));
+        }
+        {
+            Instant instant = Instant.from(formatter.parse("2019-05-06T14:52:37.123Z"));
+            assertThat(instant.getNano(), is(123_000_000));
+        }
+        {
+            Instant instant = Instant.from(formatter.parse("2019-05-06T14:52:37.1234Z"));
+            assertThat(instant.getNano(), is(123_400_000));
+        }
+        {
+            Instant instant = Instant.from(formatter.parse("2019-05-06T14:52:37.12345Z"));
+            assertThat(instant.getNano(), is(123_450_000));
+        }
+        {
+            Instant instant = Instant.from(formatter.parse("2019-05-06T14:52:37.123456Z"));
+            assertThat(instant.getNano(), is(123_456_000));
+        }
+        {
+            Instant instant = Instant.from(formatter.parse("2019-05-06T14:52:37.1234567Z"));
+            assertThat(instant.getNano(), is(123_456_700));
+        }
+        {
+            Instant instant = Instant.from(formatter.parse("2019-05-06T14:52:37.12345678Z"));
+            assertThat(instant.getNano(), is(123_456_780));
+        }
+        {
+            Instant instant = Instant.from(formatter.parse("2019-05-06T14:52:37.123456789Z"));
+            assertThat(instant.getNano(), is(123_456_789));
+        }
+    }
 }