fix(openmetrics): fix label names and add test for them

lib/private/OpenMetrics/Exporters/InstanceInfo.php

@@ -52,8 +52,8 @@ public function metrics(): Generator {
 		yield new Metric(
 			1,
 			[
-				'full version' => $this->serverVersion->getHumanVersion(),
-				'major version' => (string)$this->serverVersion->getVersion()[0],
+				'full_version' => $this->serverVersion->getHumanVersion(),
+				'major_version' => (string)$this->serverVersion->getVersion()[0],
 				'build' => $this->serverVersion->getBuild(),
 				'installed' => $this->systemConfig->getValue('installed', false) ? '1' : '0',
 			],

tests/lib/OpenMetrics/Exporters/ExporterTestCase.php

@@ -25,12 +25,30 @@ protected function setUp(): void {
 		$this->metrics = iterator_to_array($this->exporter->metrics());
 	}
 
-	public function testNotEmptyData() {
+	public function testNotEmptyData(): void {
 		$this->assertNotEmpty($this->exporter->name());
 		$this->assertNotEmpty($this->metrics);
 	}
 
-	protected function assertLabelsAre(array $expectedLabels) {
+	public function testValidNames(): void {
+		$labelNames = [];
+		foreach ($this->metrics as $metric) {
+			foreach ($metric->labels as $label => $value) {
+				$labelNames[$label] = $label;
+			}
+		}
+
+		if (empty($labelNames)) {
+			$this->expectNotToPerformAssertions();
+			return;
+		}
+		foreach ($labelNames as $label) {
+			$this->assertMatchesRegularExpression('/^[a-z_][a-z0-9_]*$/i', $label);
+		}
+
+	}
+
+	protected function assertLabelsAre(array $expectedLabels): void {
 		$foundLabels = [];
 		foreach ($this->metrics as $metric) {
 			$foundLabels[] = $metric->labels;

tests/lib/OpenMetrics/Exporters/InstanceInfoTest.php

@@ -33,8 +33,8 @@ public function testMetrics(): void {
 		$this->assertCount(1, $this->metrics);
 		$metric = array_pop($this->metrics);
 		$this->assertSame([
-			'full version' => '33.13.17 Gold',
-			'major version' => '33',
+			'full_version' => '33.13.17 Gold',
+			'major_version' => '33',
 			'build' => 'dev',
 			'installed' => '0',
 		], $metric->labels);