[+] Metrics (#95)

* Add actuator and micrometer

* update

* [-] Remove unused import

* [O] Make code less verbose

* format

* refactor

---------

Co-authored-by: Azalea <22280294+hykilpikonna@users.noreply.github.com>

src/main/java/icu/samnyan/aqua/spring/Metrics.kt

@@ -0,0 +1,45 @@
+package icu.samnyan.aqua.spring
+
+import io.micrometer.core.instrument.Counter
+import io.micrometer.core.instrument.Timer
+import java.util.concurrent.ConcurrentHashMap
+import io.micrometer.core.instrument.Metrics as MMetrics
+
+object Metrics {
+    fun counter(metricName: String, vararg pairs: Pair<String, Any>): Counter {
+        val expandedLabels = expandLabels(*pairs)
+        return cache.computeIfAbsent(MetricCacheKey(Counter::class.java, metricName, expandedLabels)) {
+            Counter
+                .builder(metricName)
+                .tags(*expandedLabels)
+                .register(MMetrics.globalRegistry)
+        } as Counter
+    }
+
+    fun timer(metricName: String, vararg pairs: Pair<String, Any>): Timer {
+        val expandedLabels = expandLabels(*pairs)
+        return cache.computeIfAbsent(MetricCacheKey(Timer::class.java, metricName, expandedLabels)) {
+            Timer
+                .builder(metricName)
+                .publishPercentiles(0.5, 0.75, 0.90, 0.95, 0.99)
+                .tags(*expandedLabels)
+                .register(MMetrics.globalRegistry)
+        } as Timer
+    }
+
+    private data class MetricCacheKey(
+        val type: Class<*>,
+        val metricName: String,
+        val expandedLabels: Array<String>,
+    )
+
+    private val cache = ConcurrentHashMap<MetricCacheKey, Any>()
+
+    private fun expandLabels(vararg pairs: Pair<String, Any>): Array<String> {
+        return pairs
+            .flatMap {
+                listOf(it.first, it.second.toString())
+            }
+            .toTypedArray()
+    }
+}