Add IConnectionMultiplexer.ServerMaintenanceEvent (#2306)

Fixing a few requests here.

Author: NickCraver

docs/ReleaseNotes.md

@@ -10,6 +10,7 @@ Current package versions:
 
 - Fix [#1520](https://github.com/StackExchange/StackExchange.Redis/issues/1520) & [#1660](https://github.com/StackExchange/StackExchange.Redis/issues/1660): When `MOVED` is encountered from a cluster, a reconfigure will happen proactively to react to cluster changes ASAP ([#2286 by NickCraver](https://github.com/StackExchange/StackExchange.Redis/pull/2286))
 - Fix [#2249](https://github.com/StackExchange/StackExchange.Redis/issues/2249): Properly handle a `fail` state (new `ClusterNode.IsFail` property) for `CLUSTER NODES` and expose `fail?` as a property (`IsPossiblyFail`) as well ([#2288 by NickCraver](https://github.com/StackExchange/StackExchange.Redis/pull/2288))
+- Adds: `IConnectionMultiplexer.ServerMaintenanceEvent` (was on `ConnectionMultiplexer` but not the interface) ([#2306 by NickCraver](https://github.com/StackExchange/StackExchange.Redis/pull/2306))
 
 
 ## 2.6.80

src/StackExchange.Redis/Interfaces/IConnectionMultiplexer.cs

@@ -3,6 +3,7 @@
 using System.IO;
 using System.Net;
 using System.Threading.Tasks;
+using StackExchange.Redis.Maintenance;
 using StackExchange.Redis.Profiling;
 
 namespace StackExchange.Redis
@@ -115,6 +116,11 @@ public interface IConnectionMultiplexer : IDisposable, IAsyncDisposable
         /// </summary>
         event EventHandler<EndPointEventArgs> ConfigurationChangedBroadcast;
 
+        /// <summary>
+        /// Raised when server indicates a maintenance event is going to happen.
+        /// </summary>
+        event EventHandler<ServerMaintenanceEvent> ServerMaintenanceEvent;
+
         /// <summary>
         /// Gets all endpoints defined on the multiplexer.
         /// </summary>

src/StackExchange.Redis/PublicAPI/PublicAPI.Shipped.txt

@@ -492,6 +492,7 @@ StackExchange.Redis.IConnectionMultiplexer.PublishReconfigure(StackExchange.Redi
 StackExchange.Redis.IConnectionMultiplexer.PublishReconfigureAsync(StackExchange.Redis.CommandFlags flags = StackExchange.Redis.CommandFlags.None) -> System.Threading.Tasks.Task<long>!
 StackExchange.Redis.IConnectionMultiplexer.RegisterProfiler(System.Func<StackExchange.Redis.Profiling.ProfilingSession!>! profilingSessionProvider) -> void
 StackExchange.Redis.IConnectionMultiplexer.ResetStormLog() -> void
+StackExchange.Redis.IConnectionMultiplexer.ServerMaintenanceEvent -> System.EventHandler<StackExchange.Redis.Maintenance.ServerMaintenanceEvent!>!
 StackExchange.Redis.IConnectionMultiplexer.StormLogThreshold.get -> int
 StackExchange.Redis.IConnectionMultiplexer.StormLogThreshold.set -> void
 StackExchange.Redis.IConnectionMultiplexer.TimeoutMilliseconds.get -> int

tests/StackExchange.Redis.Tests/Helpers/SharedConnectionFixture.cs

@@ -5,6 +5,7 @@
 using System.Net;
 using System.Threading;
 using System.Threading.Tasks;
+using StackExchange.Redis.Maintenance;
 using StackExchange.Redis.Profiling;
 using Xunit;
 
@@ -118,6 +119,12 @@ public event EventHandler<HashSlotMovedEventArgs> HashSlotMoved
             remove => _inner.HashSlotMoved -= value;
         }
 
+        public event EventHandler<ServerMaintenanceEvent> ServerMaintenanceEvent
+        {
+            add => _inner.ServerMaintenanceEvent += value;
+            remove => _inner.ServerMaintenanceEvent -= value;
+        }
+
         public void Close(bool allowCommandsToComplete = true) => _inner.Close(allowCommandsToComplete);
 
         public Task CloseAsync(bool allowCommandsToComplete = true) => _inner.CloseAsync(allowCommandsToComplete);