feat(carreirs): implement environment context getter setter (#1668)

* feat(carreirs): implement environment context getter setter

Closes #1566

* tests(carrier): add EnvironmentGetterSetter missing test

* fix(carrier): modify keys empty env condition

* test: use OpenTelemetry\Tests\TestState trait to handle environment

* docs: add trace context and baggage environment carrier example

* refactor: clean up baggage and trace context examples by removing unused span logic

* test: fix Psalm static analysis failures

Author: ycchuang99

Propagation/EnvironmentGetterSetter.php

@@ -0,0 +1,61 @@
+<?php
+
+declare(strict_types=1);
+
+namespace OpenTelemetry\Context\Propagation;
+
+use InvalidArgumentException;
+
+/**
+ * @see https://github.com/open-telemetry/opentelemetry-specification/blob/v1.44.0/specification/context/env-carriers.md
+ *
+ * Default implementation of {@see ExtendedPropagationGetterInterface} and {@see PropagationSetterInterface}.
+ * This type uses environment variables as a carrier for context propagation.
+ * It is provided to {@see TextMapPropagatorInterface::inject()} or {@see TextMapPropagatorInterface::extract()}.
+ */
+final class EnvironmentGetterSetter implements ExtendedPropagationGetterInterface, PropagationSetterInterface
+{
+    private static ?self $instance = null;
+
+    public static function getInstance(): self
+    {
+        return self::$instance ??= new self();
+    }
+
+    #[\Override]
+    public function keys($carrier): array
+    {
+        $envs = getenv();
+        if (!is_array($envs) || $envs === []) {
+            return [];
+        }
+
+        return array_map('strtolower', array_keys($envs));
+    }
+
+    #[\Override]
+    public function get($carrier, string $key): ?string
+    {
+        $value = getenv(strtoupper($key));
+
+        return is_string($value) ? $value : null;
+    }
+
+    #[\Override]
+    public function getAll($carrier, string $key): array
+    {
+        $value = getenv(strtoupper($key));
+
+        return is_string($value) ? [$value] : [];
+    }
+
+    #[\Override]
+    public function set(&$carrier, string $key, string $value): void
+    {
+        if ($key === '') {
+            throw new InvalidArgumentException('Unable to set value with an empty key');
+        }
+
+        putenv(sprintf('%s=%s', strtoupper($key), $value));
+    }
+}