Avoid dictionary allocation in observable metric callbacks

Change GetMetrics() to return IEnumerable of tuples via Select instead
of allocating a Dictionary via ToDictionary on every collection cycle.
Also simplify GetConnectionMax to use Select instead of SelectMany.

Author: fdcastel

src/FirebirdSql.Data.FirebirdClient/FirebirdClient/FbConnectionPoolManager.cs

@@ -224,11 +224,8 @@ internal void ClearPool(ConnectionString connectionString)
 		}
 	}
 
-	internal Dictionary<string, (int idleCount, int busyCount, int maxSize)> GetMetrics() =>
-		_pools.ToDictionary(
-			kvp => kvp.Key,
-			kvp => (kvp.Value.AvailableCount, kvp.Value.BusyCount, kvp.Value.MaxSize)
-		);
+	internal IEnumerable<(string poolName, int idleCount, int busyCount, int maxSize)> GetMetrics() =>
+		_pools.Select(kvp => (kvp.Key, kvp.Value.AvailableCount, kvp.Value.BusyCount, kvp.Value.MaxSize));
 
 	public void Dispose()
 	{

src/FirebirdSql.Data.FirebirdClient/Metrics/FbMetricsStore.cs

@@ -97,29 +97,26 @@ internal static void ConnectionOpened(long startedAtTicks, string poolName)
 
 		static IEnumerable<Measurement<int>> GetConnectionCount() =>
 			FbConnectionPoolManager.Instance.GetMetrics()
-				.SelectMany(kvp => new List<Measurement<int>>
+				.SelectMany(m => new List<Measurement<int>>
 				{
 					new(
-						kvp.Value.idleCount,
-						new(ConnectionPoolNameAttributeName, kvp.Key),
+						m.idleCount,
+						new(ConnectionPoolNameAttributeName, m.poolName),
 						new(ConnectionStateAttributeName, ConnectionStateIdleValue)
 					),
 
 					new(
-						kvp.Value.busyCount,
-						new(ConnectionPoolNameAttributeName, kvp.Key),
+						m.busyCount,
+						new(ConnectionPoolNameAttributeName, m.poolName),
 						new(ConnectionStateAttributeName, ConnectionStateUsedValue)
 					),
 				});
 
 		static IEnumerable<Measurement<int>> GetConnectionMax() =>
 			FbConnectionPoolManager.Instance.GetMetrics()
-				.SelectMany(kvp => new List<Measurement<int>>
-				{
-					new(
-						kvp.Value.maxSize,
-						[new(ConnectionPoolNameAttributeName, kvp.Key)]
-					),
-				});
+				.Select(m => new Measurement<int>(
+					m.maxSize,
+					[new(ConnectionPoolNameAttributeName, m.poolName)]
+				));
 	}
 }