Add standardized property to distinguish a group of applications

This adds a property to provide some indicator that a set of
applications are part of a larger "business application" so that they
can be viewed in metrics, portals, traces and more.

See gh-39957

spring-boot-project/spring-boot-docs/src/docs/antora/modules/how-to/pages/properties-and-configuration.adoc

@@ -204,6 +204,7 @@ The preceding example YAML corresponds to the following `application.properties`
 [source,properties,subs="verbatim",configprops]
 ----
 spring.application.name=cruncher
+spring.application.group=crunchGroup
 spring.datasource.driver-class-name=com.mysql.jdbc.Driver
 spring.datasource.url=jdbc:mysql://localhost/test
 server.port=9000

spring-boot-project/spring-boot-docs/src/docs/antora/modules/reference/pages/actuator/tracing.adoc

@@ -88,9 +88,10 @@ logging:
   pattern:
     correlation: "[${spring.application.name:},%X{traceId:-},%X{spanId:-}] "
   include-application-name: false
+  include-application-group: false
 ----
 
-NOTE: In the example above, configprop:logging.include-application-name[] is set to `false` to avoid the application name being duplicated in the log messages (configprop:logging.pattern.correlation[] already contains it).
+NOTE: In the example above, configprop:logging.include-application-name[] and configprop:logging.include-application-group[] is set to `false` to avoid the application name being duplicated in the log messages (configprop:logging.pattern.correlation[] already contains it).
 It's also worth mentioning that configprop:logging.pattern.correlation[] contains a trailing space so that it is separated from the logger name that comes right after it by default.
 
 

spring-boot-project/spring-boot-docs/src/docs/antora/modules/reference/pages/features/logging.adoc

@@ -34,6 +34,7 @@ The following items are output:
 * Process ID.
 * A `---` separator to distinguish the start of actual log messages.
 * Application name: Enclosed in square brackets (logged by default only if configprop:spring.application.name[] is set)
+* Application group: Enclosed in square brackets (logged by default only if configprop:spring.application.group[] is set)
 * Thread name: Enclosed in square brackets (may be truncated for console output).
 * Correlation ID: If tracing is enabled (not shown in the sample above)
 * Logger name: This is usually the source class name (often abbreviated).
@@ -43,6 +44,7 @@ NOTE: Logback does not have a `FATAL` level.
 It is mapped to `ERROR`.
 
 TIP: If you have a configprop:spring.application.name[] property but don't want it logged you can set configprop:logging.include-application-name[] to `false`.
+TIP: If you have a configprop:spring.application.group[] property but don't want it logged you can set configprop:logging.include-application-group[] to `false`.
 
 
 
@@ -544,12 +546,13 @@ The following listing shows three sample profiles:
 If you want to refer to properties from your Spring `Environment` within your Log4j2 configuration you can use `spring:` prefixed https://logging.apache.org/log4j/2.x/manual/lookups.html[lookups].
 Doing so can be useful if you want to access values from your `application.properties` file in your Log4j2 configuration.
 
-The following example shows how to set a Log4j2 property named `applicationName` that reads `spring.application.name` from the Spring `Environment`:
+The following example shows how to set a Log4j2 property named `applicationName` and `applicationGroup` that reads `spring.application.name` and `spring.application.group` from the Spring `Environment`:
 
 [source,xml]
 ----
 <Properties>
 	<Property name="applicationName">${spring:spring.application.name}</Property>
+	<Property name="applicationProperty">${spring:spring.application.property}</Property>
 </Properties>
 ----