Set default timeout for Trace Exporter to 15s. (#7252)

## Summary of changes
Add new binding to set the trace exporter connection timeout.
## Reason for change
Trace exporter was setting a 3s connection timeout which probably was
insufficient for some test cases. This version use the new bindings to
increase the time up to 15 seconds.

Author: hoolioh

tracer/src/Datadog.Trace/LibDatadog/NativeInterop.cs

@@ -93,6 +93,9 @@ internal static class Config
 
         [DllImport(DllName, EntryPoint = "ddog_trace_exporter_config_set_client_computed_stats")]
         internal static extern TraceExporterErrorHandle SetClientComputedStats(SafeHandle config, bool clientComputedStats);
+
+        [DllImport(DllName, EntryPoint = "ddog_trace_exporter_config_set_connection_timeout")]
+        internal static extern TraceExporterErrorHandle SetConnectionTimeout(SafeHandle config, ulong timeout_ms);
     }
 
     internal static class Logger

tracer/src/Datadog.Trace/LibDatadog/TraceExporterConfiguration.cs

@@ -151,6 +151,15 @@ public bool ClientComputedStats
         }
     }
 
+    public ulong ConnectionTimeoutMs
+    {
+        init
+        {
+            using var error = NativeInterop.Config.SetConnectionTimeout(this, value);
+            error.ThrowIfError();
+        }
+    }
+
     protected override bool ReleaseHandle()
     {
         if (_telemetryConfigPtr != IntPtr.Zero)

tracer/src/Datadog.Trace/TracerManagerFactory.cs

@@ -415,7 +415,8 @@ internal static IApi GetApi(TracerSettings settings, IDogStatsd statsd, Action<D
                         LanguageInterpreter = frameworkDescription.Name,
                         ComputeStats = settings.StatsComputationEnabled,
                         TelemetryClientConfiguration = telemetryClientConfiguration,
-                        ClientComputedStats = clientComputedStats
+                        ClientComputedStats = clientComputedStats,
+                        ConnectionTimeoutMs = 15_000
                     };
 
                     return new TraceExporter(configuration, updateSampleRates);