Polish StartupStep "tags" method

Renaming the `tags` method to `getTags` for overall consistency, and
fixing the Javadoc to mention that this returns an immutable collection.

Closes gh-25678

spring-core/src/main/java/org/springframework/core/metrics/DefaultApplicationStartup.java

@@ -57,7 +57,7 @@ public Long getParentId() {
 		}
 
 		@Override
-		public Tags tags() {
+		public Tags getTags() {
 			return this.TAGS;
 		}
 

spring-core/src/main/java/org/springframework/core/metrics/StartupStep.java

@@ -76,7 +76,7 @@ public interface StartupStep {
 	/**
 	 * Return the {@link Tag} collection for this step.
 	 */
-	Tags tags();
+	Tags getTags();
 
 	/**
 	 * Record the state of the step and possibly other metrics like execution time.
@@ -86,7 +86,7 @@ public interface StartupStep {
 
 
 	/**
-	 * Mutable collection of {@link Tag}.
+	 * Immutable collection of {@link Tag}.
 	 */
 	interface Tags extends Iterable<Tag> {
 	}

spring-core/src/main/java/org/springframework/core/metrics/jfr/FlightRecorderStartupStep.java

@@ -77,7 +77,7 @@ public StartupStep tag(String key, Supplier<String> value) {
 	}
 
 	@Override
-	public Tags tags() {
+	public Tags getTags() {
 		return this.tags;
 	}