feat: Add labels support to InfoMetric.

jicoco-metrics/src/main/kotlin/org/jitsi/metrics/InfoMetric.kt

@@ -24,21 +24,49 @@ import io.prometheus.client.Info
  * In the Prometheus exposition format, these are shown as labels of either a custom metric (OpenMetrics)
  * or a [Gauge][io.prometheus.client.Gauge] (0.0.4 plain text).
  */
-class InfoMetric(
+class InfoMetric @JvmOverloads constructor(
     /** the name of this metric */
     override val name: String,
     /** the description of this metric */
     help: String,
     /** the namespace (prefix) of this metric */
     namespace: String,
     /** the value of this info metric */
-    internal val value: String
+    internal val value: String = "",
+    /** Label names for this metric */
+    val labelNames: List<String> = emptyList()
 ) : Metric<String>() {
-    private val info = Info.build(name, help).namespace(namespace).create().apply { info(name, value) }
+    private val info = run {
+        val builder = Info.build(name, help).namespace(namespace)
+        if (labelNames.isNotEmpty()) {
+            builder.labelNames(*labelNames.toTypedArray())
+        }
+        builder.create().apply {
+            if (labelNames.isEmpty()) {
+                info(name, value)
+            }
+        }
+    }
 
-    override fun get() = value
+    override fun get() = if (labelNames.isEmpty()) value else throw UnsupportedOperationException()
+    fun get(labels: List<String> = emptyList()) =
+        if (labels.isEmpty()) value else info.labels(*labels.toTypedArray()).get()[name]
 
-    override fun reset() = info.info(name, value)
+    override fun reset() = if (labelNames.isEmpty()) info.info(name, value) else info.clear()
 
     override fun register(registry: CollectorRegistry) = this.also { registry.register(info) }
+
+    /** Remove the child with the given labels (the metric with those labels will stop being emitted) */
+    fun remove(labels: List<String> = emptyList()) = synchronized(info) {
+        if (labels.isNotEmpty()) {
+            info.remove(*labels.toTypedArray())
+        }
+    }
+
+    fun set(labels: List<String>, value: String) {
+        if (labels.isNotEmpty()) {
+            info.labels(*labels.toTypedArray()).info(name, value)
+        }
+    }
+    internal fun collect() = info.collect()
 }

jicoco-metrics/src/main/kotlin/org/jitsi/metrics/MetricsContainer.kt

@@ -238,15 +238,18 @@ open class MetricsContainer @JvmOverloads constructor(
         /** the description of the metric */
         help: String,
         /** the value of the metric */
-        value: String
+        value: String,
+        /** Label names for this metric. If non-empty, the initial value must be 0 and all get/update calls MUST
+         * specify values for the labels. Calls to simply get() or inc() will fail with an exception. */
+        labelNames: List<String> = emptyList()
     ): InfoMetric {
         if (metrics.containsKey(name)) {
             if (checkForNameConflicts) {
                 throw RuntimeException("Could not register metric '$name'. A metric with that name already exists.")
             }
             return metrics[name] as InfoMetric
         }
-        return InfoMetric(name, help, namespace, value).apply { metrics[name] = register(registry) }
+        return InfoMetric(name, help, namespace, value, labelNames).apply { metrics[name] = register(registry) }
     }
 
     fun registerHistogram(

jicoco-metrics/src/test/kotlin/org/jitsi/metrics/MetricTest.kt

@@ -325,6 +325,34 @@ class MetricTest : ShouldSpec() {
                     should("return the correct value") { get() shouldBe value }
                 }
             }
+            context("With labels") {
+                with(InfoMetric("testInfo", "Help", namespace, labelNames = listOf("l1", "l2"))) {
+                    val labels = listOf("A", "A")
+                    val labels2 = listOf("A", "B")
+                    val labels3 = listOf("B", "B")
+
+                    shouldThrow<Exception> { get() }
+
+                    get(labels) shouldBe null
+                    get(labels2) shouldBe null
+
+                    set(labels, "AA")
+                    get(labels) shouldBe "AA"
+                    get(labels2) shouldBe null
+
+                    set(labels3, "BB")
+                    get(labels) shouldBe "AA"
+                    get(labels3) shouldBe "BB"
+
+                    collect()[0].samples.size shouldBe 3
+                    remove(labels2)
+                    // Down to two sets of labels
+                    collect()[0].samples.size shouldBe 2
+                    get(labels) shouldBe "AA"
+                    get(labels2) shouldBe null
+                    get(labels3) shouldBe "BB"
+                }
+            }
         }
         context("HistogramMetric") {
             val namespace = "namespace"