LowResourceMonitor.getReasons should include detailed reason instead of hard-coded message

[jluehe]

Jetty version(s)
10.0.13

Description
LowResourceMonitor.getReasons returns a hard-coded string, namely the string representation of the LowResourceCheck that is low on resources. It should contain the detailed low-resource reason instead, which would be more useful.

For example, it should return Server low on threads: x, idleThreads: y vs the current Check if the ThreadPool from monitored connectors are lowOnThreads and if all connections number is higher than the allowed maxConnection.

Proposed diffs:

diff --git a/jetty-server/src/main/java/org/eclipse/jetty/server/LowResourceMonitor.java b/jetty-server/src/main/java/org/eclipse/jetty/server/LowResourceMonitor.java
index 07c7ab427d..7529404b17 100644
--- a/jetty-server/src/main/java/org/eclipse/jetty/server/LowResourceMonitor.java
+++ b/jetty-server/src/main/java/org/eclipse/jetty/server/LowResourceMonitor.java
@@ -280,7 +280,7 @@ public class LowResourceMonitor extends ContainerLifeCycle
         {
             if (lowResourceCheck.isLowOnResources())
             {
-                reasons = lowResourceCheck.toString();
+                reasons = lowResourceCheck.getReason();
                 break;
             }
         }

If needed, the LowResourceCheck string representation could be appended to the reason.

[jluehe]

On a separate note, it looks like even if multiple LowResourceChecks are low on resources, only one of them will contribute to the LowResourceMonitor reasons (plural): https://github.com/eclipse/jetty.project/blob/jetty-10.0.13/jetty-server/src/main/java/org/eclipse/jetty/server/LowResourceMonitor.java#L279-L286. Perhaps they should all be concatenated?