fixing some psalm 5 complaints (#1110)

Whilst investigating upgrading to psalm 5, I notice that it generates a lot of new complaints. This fixes the ones that looked legit and
require an API change, or were trivial. Does not upgrade to psalm 5 yet, though. That's a bigger job for another day.

Author: brettmc

Signals.php

@@ -4,9 +4,7 @@
 
 namespace OpenTelemetry\API;
 
-use InvalidArgumentException;
-
-class Signals
+interface Signals
 {
     /** @var string  */
     public const TRACE = 'trace';
@@ -20,11 +18,4 @@ class Signals
         self::METRICS,
         self::LOGS,
     ];
-
-    public static function validate(string $signal): void
-    {
-        if (!in_array($signal, self::SIGNALS)) {
-            throw new InvalidArgumentException('Unknown signal: ' . $signal);
-        }
-    }
 }

Trace/SpanKind.php

@@ -7,26 +7,11 @@
 /**
  * @see https://github.com/open-telemetry/opentelemetry-specification/blob/v1.6.1/specification/trace/api.md#spankind
  */
-final class SpanKind
+interface SpanKind
 {
     public const KIND_INTERNAL = 0;
     public const KIND_CLIENT = 1;
     public const KIND_SERVER = 2;
     public const KIND_PRODUCER = 3;
     public const KIND_CONSUMER = 4;
-
-    public static function getChoices(): array
-    {
-        return [
-            self::KIND_INTERNAL,
-            self::KIND_CLIENT,
-            self::KIND_SERVER,
-            self::KIND_PRODUCER,
-            self::KIND_CONSUMER,
-        ];
-    }
-
-    private function __construct()
-    {
-    }
 }

Trace/StatusCode.php

@@ -7,22 +7,9 @@
 /**
  * @see https://github.com/open-telemetry/opentelemetry-specification/blob/v1.6.1/specification/trace/api.md#set-status
  */
-final class StatusCode
+interface StatusCode
 {
     public const STATUS_UNSET = 'Unset';
     public const STATUS_OK = 'Ok';
     public const STATUS_ERROR = 'Error';
-
-    public function getChoices(): array
-    {
-        return [
-            self::STATUS_UNSET,
-            self::STATUS_OK,
-            self::STATUS_ERROR,
-        ];
-    }
-
-    private function __construct()
-    {
-    }
 }