Remove remaining '#if NET6_0_OR_GREATER' directives on OTLP Trace export code paths.

Note: The OTLP Metrics export will need to be updated to be supported below net6.0 for trace metrics to be supported on all runtimes.

Author: zacharycmontoya

tracer/src/Datadog.Trace/Agent/ApiOtlp.cs

@@ -3,8 +3,6 @@
 // This product includes software developed at Datadog (https://www.datadoghq.com/). Copyright 2017 Datadog, Inc.
 // </copyright>
 
-#if NET6_0_OR_GREATER
-
 using System;
 using System.Collections.Generic;
 using System.IO;
@@ -37,7 +35,9 @@ internal sealed class ApiOtlp : IApi
         private readonly Uri _statsEndpoint; // This endpoint is passed for the _sendStats callback, but otherwise unused
         private readonly SendCallback<SendStatsState> _sendStats;
         private readonly SendCallback<SendTracesState> _sendTraces;
+#if NET6_0_OR_GREATER
         private readonly Datadog.Trace.OpenTelemetry.Metrics.OtlpExporter _metricsExporter;
+#endif
 
         public ApiOtlp(IApiRequestFactory apiRequestFactory, TracerSettings settings, ExporterSettings exporterSettings, IDatadogLogger log = null)
         {
@@ -54,7 +54,9 @@ public ApiOtlp(IApiRequestFactory apiRequestFactory, TracerSettings settings, Ex
             _statsEndpoint = exporterSettings.OtlpMetricsEndpoint;
             _log.Debug("Using traces endpoint {TracesEndpoint}", _tracesEndpoint.ToString());
 
+#if NET6_0_OR_GREATER
             _metricsExporter = new Datadog.Trace.OpenTelemetry.Metrics.OtlpExporter(settings, exporterSettings);
+#endif
         }
 
         private delegate Task<SendResult> SendCallback<T>(IApiRequest request, bool isFinalTry, T state);
@@ -309,4 +311,3 @@ public SendStatsState(StatsBuffer stats, long bucketDuration, int tracerObfuscat
         }
     }
 }
-#endif

tracer/src/Datadog.Trace/Agent/ManagedApiOtlp.cs

@@ -3,8 +3,6 @@
 // This product includes software developed at Datadog (https://www.datadoghq.com/). Copyright 2017 Datadog, Inc.
 // </copyright>
 
-#if NET6_0_OR_GREATER
-
 #nullable enable
 
 using System;
@@ -50,5 +48,3 @@ public Task<bool> SendTracesAsync(ArraySegment<byte> traces, int numberOfTraces,
     public Task<bool> SendStatsAsync(StatsBuffer stats, long bucketDuration, int tracerObfuscationVersion)
         => Volatile.Read(ref _api).SendStatsAsync(stats, bucketDuration, tracerObfuscationVersion);
 }
-
-#endif

tracer/src/Datadog.Trace/TracerManagerFactory.cs

@@ -293,15 +293,13 @@ protected virtual ISpanSampler GetSpanSampler(TracerSettings settings)
 
         protected virtual IAgentWriter GetAgentWriter(TracerSettings settings, IStatsdManager statsd, Action<Dictionary<string, float>> updateSampleRates, Action<string> updateConfigHash, IDiscoveryService discoveryService, TelemetrySettings telemetrySettings)
         {
-#if NET6_0_OR_GREATER
             if (settings.Manager.InitialExporterSettings.TracesEncoding is TracesEncoding.OtlpProtobuf or TracesEncoding.OtlpJson)
             {
                 var otlpApi = new ManagedApiOtlp(settings);
                 var otlpStatsAggregator = StatsAggregator.Create(otlpApi, settings, discoveryService, isOtlp: true);
                 return new AgentWriter(otlpApi, otlpStatsAggregator, statsd, settings);
             }
 
-#endif
             // Currently we assume this _can't_ toggle at runtime, may need to revisit this if that changes
             IApi api = settings.DataPipelineEnabled && ManagedTraceExporter.TryCreateTraceExporter(settings, updateSampleRates, telemetrySettings, out var traceExporter)
                            ? traceExporter