Do not inject DSM legacy headers by default (#7351)

## Summary of changes
Reversed the default state of `IsDataStreamsLegacyHeadersEnabled` for
`enabled by default` state.

## Reason for change
See [ticket](https://datadoghq.atlassian.net/browse/APMS-16734)

Author: kr-igor

tracer/src/Datadog.Trace/Configuration/ConfigurationKeys.cs

@@ -963,7 +963,7 @@ internal static class DataStreamsMonitoring
 
             /// <summary>
             /// Configuration key for enabling legacy binary headers in Data Streams Monitoring.
-            /// Default is true.
+            /// false by default if DSM is in default state, true otherwise
             /// </summary>
              /// <see cref="TracerSettings.IsDataStreamsLegacyHeadersEnabled"/>
             public const string LegacyHeadersEnabled = "DD_DATA_STREAMS_LEGACY_HEADERS";

tracer/src/Datadog.Trace/Configuration/TracerSettings.cs

@@ -660,9 +660,10 @@ internal TracerSettings(IConfigurationSource? source, IConfigurationTelemetry te
                                                     .WithKeys(ConfigurationKeys.DataStreamsMonitoring.Enabled)
                                                     .AsBool() == null;
 
+            // no legacy headers if we are in "enbaled by default" state
             IsDataStreamsLegacyHeadersEnabled = config
                                                .WithKeys(ConfigurationKeys.DataStreamsMonitoring.LegacyHeadersEnabled)
-                                               .AsBool(true);
+                                               .AsBool(!_isDataStreamsMonitoringInDefaultState);
 
             IsRareSamplerEnabled = config
                                   .WithKeys(ConfigurationKeys.RareSamplerEnabled)

tracer/test/Datadog.Trace.ClrProfiler.Managed.Tests/AutoInstrumentation/AWS/Kinesis/ContextPropagationTests.cs

@@ -148,7 +148,6 @@ public async Task InjectTraceIntoRecords_WithJsonString_AddsTraceContext()
         extractedTraceContext["x-datadog-parent-id"].Should().Be(scope?.Span.SpanId.ToString());
         extractedTraceContext["x-datadog-trace-id"].Should().Be(scope?.Span.TraceId.ToString());
         extractedTraceContext["dd-pathway-ctx-base64"].As<Newtonsoft.Json.Linq.JArray>().Should().HaveCount(1);
-        extractedTraceContext["dd-pathway-ctx"].As<Newtonsoft.Json.Linq.JArray>().Should().HaveCount(1);
     }
 
     [Fact]
@@ -203,7 +202,6 @@ public async Task InjectTraceIntoData_WithJsonString_AddsTraceContext()
         extractedTraceContext["x-datadog-parent-id"].Should().Be(scope?.Span.SpanId.ToString());
         extractedTraceContext["x-datadog-trace-id"].Should().Be(scope?.Span.TraceId.ToString());
         extractedTraceContext["dd-pathway-ctx-base64"].As<Newtonsoft.Json.Linq.JArray>().Should().HaveCount(1);
-        extractedTraceContext["dd-pathway-ctx"].As<Newtonsoft.Json.Linq.JArray>().Should().HaveCount(1);
     }
 
     [Fact]

tracer/test/Datadog.Trace.Tests/Configuration/TracerSettingsTests.cs

@@ -979,7 +979,7 @@ public void IsDataStreamsMonitoringEnabled(string value, bool expected)
         }
 
         [Theory]
-        [MemberData(nameof(BooleanTestCases), true)]
+        [MemberData(nameof(BooleanTestCases), false)]
         public void IsDataStreamsLegacyHeadersEnabled(string value, bool expected)
         {
             var source = CreateConfigurationSource((ConfigurationKeys.DataStreamsMonitoring.LegacyHeadersEnabled, value));