fix: enhance thread safety in Metrics library to prevent concurrent modification issues

Author: hjgraca

libraries/src/AWS.Lambda.Powertools.Metrics/Metrics.cs

@@ -535,9 +535,17 @@ private Dictionary<string, string> ListToDictionary(List<DimensionSet> dimension
         var dictionary = new Dictionary<string, string>();
         try
         {
-            return dimensions != null
-                ? new Dictionary<string, string>(dimensions.SelectMany(x => x.Dimensions))
-                : dictionary;
+            if (dimensions != null)
+            {
+                foreach (var dimensionSet in dimensions)
+                {
+                    foreach (var kvp in dimensionSet.Dimensions)
+                    {
+                        dictionary[kvp.Key] = kvp.Value;
+                    }
+                }
+            }
+            return dictionary;
         }
         catch (Exception e)
         {
@@ -633,20 +641,34 @@ private static MetricDefinition GetExistingMetric(List<MetricDefinition> metrics
     {
         // Use a traditional for loop instead of LINQ to avoid enumeration issues
         // when the collection is modified concurrently
-        for (int i = 0; i < metrics.Count; i++)
+        if (metrics == null || string.IsNullOrEmpty(key))
+            return null;
+            
+        // Create a snapshot of the count to avoid issues with concurrent modifications
+        var count = metrics.Count;
+        for (int i = 0; i < count; i++)
         {
             try
             {
+                // Check bounds again in case collection was modified
+                if (i >= metrics.Count)
+                    break;
+                    
                 var metric = metrics[i];
-                if (metric?.Name == key)
+                if (metric != null && string.Equals(metric.Name, key, StringComparison.Ordinal))
                 {
                     return metric;
                 }
             }
             catch (ArgumentOutOfRangeException)
             {
                 // Collection was modified during iteration, return null to be safe
-                return null;
+                break;
+            }
+            catch (IndexOutOfRangeException)
+            {
+                // Collection was modified during iteration, return null to be safe
+                break;
             }
         }
         return null;

libraries/src/AWS.Lambda.Powertools.Metrics/Model/DimensionSet.cs

@@ -43,5 +43,16 @@ public DimensionSet(string key, string value)
     ///     Gets the dimension keys.
     /// </summary>
     /// <value>The dimension keys.</value>
-    public List<string> DimensionKeys => Dimensions.Keys.ToList();
+    public List<string> DimensionKeys 
+    { 
+        get 
+        {
+            var keys = new List<string>();
+            foreach (var key in Dimensions.Keys)
+            {
+                keys.Add(key);
+            }
+            return keys;
+        }
+    }
 }

libraries/src/AWS.Lambda.Powertools.Metrics/Model/Metadata.cs

@@ -51,7 +51,10 @@ public Metadata()
     /// </summary>
     internal void ClearMetrics()
     {
-        _metricDirective.Metrics.Clear();
+        lock (_metricDirective._lockObj)
+        {
+            _metricDirective.Metrics.Clear();
+        }
         CustomMetadata?.Clear();
     }
 
@@ -60,7 +63,10 @@ internal void ClearMetrics()
     /// </summary>
     internal void ClearNonDefaultDimensions()
     {
-        _metricDirective.Dimensions.Clear();
+        lock (_metricDirective._lockObj)
+        {
+            _metricDirective.Dimensions.Clear();
+        }
     }
 
     /// <summary>
@@ -144,7 +150,11 @@ internal void SetDefaultDimensions(List<DimensionSet> defaultDimensionSets)
     /// <returns>List of metrics stored in memory</returns>
     internal List<MetricDefinition> GetMetrics()
     {
-        return _metricDirective.Metrics;
+        // Return a snapshot to avoid concurrent modification issues during serialization
+        lock (_metricDirective._lockObj)
+        {
+            return new List<MetricDefinition>(_metricDirective.Metrics);
+        }
     }
 
     /// <summary>

libraries/src/AWS.Lambda.Powertools.Metrics/Model/MetricDefinition.cs

@@ -93,6 +93,9 @@ public MetricDefinition(string name, MetricUnit unit, List<double> values, Metri
     /// <param name="value">Metric value to add to existing key</param>
     public void AddValue(double value)
     {
-        Values.Add(value);
+        lock (Values)
+        {
+            Values.Add(value);
+        }
     }
 }