Add support for read-only metadata (#28)

Author: Falco20019

src/OpenTracing.Contrib.Grpc/GrpcTraceLogger.cs

@@ -35,7 +35,7 @@ public void ResponseHeader(Metadata metadata)
             _span.Log(new Dictionary<string, object>
             {
                 { LogFields.Event, "Response headers received" },
-                { "data", metadata }
+                { "data", metadata?.ToReadableString() }
             });
         }
 

src/OpenTracing.Contrib.Grpc/Handler/InterceptedClientHandler.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Globalization;
+using System.Linq;
 using System.Threading.Tasks;
 using Grpc.Core;
 using Grpc.Core.Interceptors;
@@ -45,10 +46,19 @@ public InterceptedClientHandler(ClientTracingConfiguration configuration, Client
 
         private CallOptions ApplyConfigToCallOptions(CallOptions callOptions)
         {
-            if (callOptions.Headers == null)
+            var headers = callOptions.Headers;
+            if (headers == null || headers.IsReadOnly)
             {
-                // Add empty metadata to options:
-                callOptions = callOptions.WithHeaders(new Metadata());
+                // Use empty, writable metadata as base:
+                var metadata = new Metadata();
+
+                // Copy elements since it was read-only if it had elements:
+                foreach (var header in headers ?? Enumerable.Empty<Metadata.Entry>())
+                {
+                    metadata.Add(header);
+                }
+
+                callOptions = callOptions.WithHeaders(metadata);
             }
 
             if (_configuration.WaitForReady && callOptions.IsWaitForReady != _configuration.WaitForReady)