Gauge implements MeterHandler

Sources/Prometheus/Gauge.swift

@@ -80,6 +80,16 @@ extension Gauge: CoreMetrics.RecorderHandler {
     }
 }
 
+extension Gauge: CoreMetrics.MeterHandler {
+    public func set(_ value: Double) {
+        self.set(to: value)
+    }
+
+    public func set(_ value: Int64) {
+        self.set(to: Double(value))
+    }
+}
+
 extension Gauge: PrometheusMetric {
     func emit(into buffer: inout [UInt8]) {
         let value = Double(bitPattern: self.atomic.load(ordering: .relaxed))

Sources/Prometheus/PrometheusMetricsFactory.swift

@@ -97,6 +97,10 @@ extension PrometheusMetricsFactory: CoreMetrics.MetricsFactory {
         }
     }
 
+    public func makeMeter(label: String, dimensions: [(String, String)]) -> CoreMetrics.MeterHandler {
+        return self.client.makeGauge(name: label, labels: dimensions)
+    }
+
     public func makeTimer(label: String, dimensions: [(String, String)]) -> CoreMetrics.TimerHandler {
         let (label, dimensions) = self.labelAndDimensionSanitizer(label, dimensions)
         let buckets = self.timeHistogramBuckets[label] ?? self.defaultTimeHistogramBuckets
@@ -128,6 +132,13 @@ extension PrometheusMetricsFactory: CoreMetrics.MetricsFactory {
         }
     }
 
+    public func destroyMeter(_ handler: CoreMetrics.MeterHandler) {
+        guard let gauge = handler as? Gauge else {
+            return
+        }
+        self.client.destroyGauge(gauge)
+    }
+
     public func destroyTimer(_ handler: CoreMetrics.TimerHandler) {
         guard let histogram = handler as? Histogram<Duration> else {
             return

Tests/PrometheusTests/PrometheusMetricsFactoryTests.swift

@@ -68,4 +68,56 @@ final class PrometheusMetricsFactoryTests: XCTestCase {
             """
         )
     }
+
+    func testMakeMeters() {
+        let client = PrometheusCollectorRegistry()
+        let factory = PrometheusMetricsFactory(client: client)
+
+        let maybeGauge = factory.makeMeter(label: "foo", dimensions: [("bar", "baz")])
+        XCTAssertNotNil(maybeGauge as? Gauge)
+
+        maybeGauge.increment(by: 1)
+        maybeGauge.decrement(by: 7)
+
+        var buffer = [UInt8]()
+        client.emit(into: &buffer)
+        XCTAssertEqual(String(decoding: buffer, as: Unicode.UTF8.self), """
+            # TYPE foo gauge
+            foo{bar="baz"} -6.0
+
+            """
+        )
+
+        // set to double value
+        maybeGauge.set(12.45)
+        buffer.removeAll(keepingCapacity: true)
+        client.emit(into: &buffer)
+        XCTAssertEqual(String(decoding: buffer, as: Unicode.UTF8.self), """
+            # TYPE foo gauge
+            foo{bar="baz"} 12.45
+
+            """
+        )
+
+        // set to int value
+        maybeGauge.set(Int64(42)) // needs explicit cast... otherwise ambigious
+        buffer.removeAll(keepingCapacity: true)
+        client.emit(into: &buffer)
+        XCTAssertEqual(String(decoding: buffer, as: Unicode.UTF8.self), """
+            # TYPE foo gauge
+            foo{bar="baz"} 42.0
+
+            """
+        )
+
+        factory.destroyMeter(maybeGauge)
+        buffer.removeAll(keepingCapacity: true)
+        client.emit(into: &buffer)
+        XCTAssertEqual(String(decoding: buffer, as: Unicode.UTF8.self), """
+            # TYPE foo gauge
+
+            """
+        )
+    }
+
 }