Allow to manually tag request metrics with exceptions

Prior to this commit, some exceptions handled at the controller or
handler function level would:

* not bubble up to the Spring Boot error handling support
* not be tagged as part of the request metrics

This situation is inconsistent because in general, exceptions handled at
the controller level can be considered as expected behavior.
Also, depending on how the exception is handled, the request metrics
might not be tagged with the exception.
This will be reconsidered in gh-23795.

This commit prepares a transition to the new situation. Developers can
now opt-in and set the handled exception as a request attribute. This
well-known attribute will be later read by the metrics support and used
for tagging the request metrics with the exception provided.

This mechanism is automatically used by the error handling support in
Spring Boot.

Closes gh-24028

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/springbootfeatures/webapplications/servlet/MyController.java

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2012-2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.docs.springbootfeatures.webapplications.servlet;
+
+import javax.servlet.http.HttpServletRequest;
+
+import org.springframework.boot.web.servlet.error.ErrorAttributes;
+import org.springframework.stereotype.Controller;
+import org.springframework.web.bind.annotation.ExceptionHandler;
+
+@Controller
+public class MyController {
+
+	@ExceptionHandler(CustomException.class)
+	String handleCustomException(HttpServletRequest request, CustomException ex) {
+		request.setAttribute(ErrorAttributes.ERROR_ATTRIBUTE, ex);
+		return "errorView";
+	}
+
+}
+// @chomp:file
+
+class CustomException extends RuntimeException {
+
+}

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/springbootfeatures/webapplications/webflux/ExceptionHandlingController.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2012-2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.docs.springbootfeatures.webapplications.webflux;
+
+import org.springframework.boot.web.reactive.error.ErrorAttributes;
+import org.springframework.stereotype.Controller;
+import org.springframework.web.bind.annotation.ExceptionHandler;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.reactive.result.view.Rendering;
+import org.springframework.web.server.ServerWebExchange;
+
+@Controller
+public class ExceptionHandlingController {
+
+	@GetMapping("/profile")
+	public Rendering userProfile() {
+		// ..
+		throw new IllegalStateException();
+		// ...
+	}
+
+	@ExceptionHandler(IllegalStateException.class)
+	Rendering handleIllegalState(ServerWebExchange exchange, IllegalStateException exc) {
+		exchange.getAttributes().putIfAbsent(ErrorAttributes.ERROR_ATTRIBUTE, exc);
+		return Rendering.view("errorView").modelAttribute("message", exc.getMessage()).build();
+	}
+
+}