Fix duration to microseconds conversion

See gh-27149

Author: marckchr

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/convert/DurationStyle.java

@@ -183,7 +183,7 @@ enum Unit {
 		/**
 		 * Microseconds.
 		 */
-		MICROS(ChronoUnit.MICROS, "us", (duration) -> duration.toMillis() * 1000L),
+		MICROS(ChronoUnit.MICROS, "us", duration -> duration.toNanos() / 1000L),
 
 		/**
 		 * Milliseconds.

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/convert/DurationStyleTests.java

@@ -234,6 +234,7 @@ void printSimpleWithoutUnitShouldPrintInMs() {
 	void printSimpleWithUnitShouldPrintInUnit() {
 		Duration duration = Duration.ofMillis(1000);
 		assertThat(DurationStyle.SIMPLE.print(duration, ChronoUnit.SECONDS)).isEqualTo("1s");
+		assertThat(DurationStyle.SIMPLE.print(Duration.ofNanos(2000), ChronoUnit.MICROS)).isEqualTo("2us");
 	}
 
 }