exclude api keys from sample app logs (#7592)

## Summary of changes

Exclude environment variables named `*API_KEY*` in
`SampleHelpers.GetDatadogEnvironmentVariables()`.

## Reason for change

Prevent logging API keys in test outputs.

## Implementation details

- In `GetDatadogEnvironmentVariables()`, filter out any environment
variable whose key contains "API_KEY" (case-insensitive)

Bonus changes:
- Changed the return type from `IEnumerable<T>` to `List<T>` since we
already materialize the list (potential tiny perf improvement when
iterating)
- Removed unnecessary `ToUpperInvariant()` call on the key and replaced
`StartsWith()` with a case-insensitive comparison

## Test coverage

None, it's a helper method used in tests.

## Other details
<!-- Fixes #{issue} -->


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Author: lucaspimentel

tracer/test/test-applications/Samples.Shared/SampleHelpers.cs

@@ -368,15 +368,15 @@ public static void TrySetExceptionOnActiveScope(Exception exception)
             SetExceptionMethod.Invoke(span, new object[] { new Exception() });
         }
 
-        public static IEnumerable<KeyValuePair<string, string>> GetDatadogEnvironmentVariables()
+        public static List<KeyValuePair<string, string>> GetDatadogEnvironmentVariables()
         {
             var prefixes = new[] { "COR_", "CORECLR_", "DD_", "DATADOG_" };
 
             var envVars = from envVar in Environment.GetEnvironmentVariables().Cast<DictionaryEntry>()
                           from prefix in prefixes
-                          let key = (envVar.Key as string)?.ToUpperInvariant()
+                          let key = (envVar.Key as string)?.ToUpperInvariant() // use ToUpperInvariant() because in .NET Framework string.Contains() doesn't have StringComparison overload
                           let value = envVar.Value as string
-                          where key.StartsWith(prefix)
+                          where key.StartsWith(prefix, StringComparison.Ordinal) && !key.Contains("API_KEY")
                           orderby key
                           select new KeyValuePair<string, string>(key, value);