Log file location should be evaluated just once

Using a random value for the logfile name caused
the logfile endpoint to return a 404 as the name
was resolved from the environment on every request.
This commit registers a bean for LogFile which is then
used by the logfile endpoint.

Fixes gh-17434

spring-boot-samples/spring-boot-sample-actuator/src/test/java/sample/actuator/EndpointsPropertiesSampleActuatorApplicationTests.java

@@ -66,6 +66,13 @@ public class EndpointsPropertiesSampleActuatorApplicationTests {
 		assertThat(entity.getBody()).contains("\"hello\":\"world\"");
 	}
 
+	@Test
+	public void logfileWithRandomName() {
+		ResponseEntity<String> entity = this.restTemplate.withBasicAuth("user", getPassword())
+				.getForEntity("/admin/logfile", String.class);
+		assertThat(entity.getStatusCode()).isEqualTo(HttpStatus.OK);
+	}
+
 	private String getPassword() {
 		return "password";
 	}

spring-boot-samples/spring-boot-sample-actuator/src/test/resources/application-endpoints.properties

@@ -1,3 +1,5 @@
 server.error.path: /oops
 management.endpoint.health.show-details: always
 management.endpoints.web.base-path: /admin
+logging.file=./target/${spring.application.instance_id}.log
+spring.application.instance_id=${random.value}