feat: Prometheus-метрики GET /metrics + ленивый ISO-8601-парсер для backoff

- GET /metrics (Prometheus text 0.0.4): llm_proxy_requests_total{model,upstream,provider,result}
  (ok/4xx/429/402/5xx/net_err/cancelled), llm_proxy_upstream_inflight,
  llm_proxy_upstream_fail_streak, llm_proxy_upstream_cooling_seconds
- учёт исходов в handleChat (каждая фейловер-попытка — отдельно)
- parseIsoDuration: PnM≈n×30d, PnY≈n×365d (kotlin.time Duration.parse не принимает Y/M)
- remainingSeconds округляет остаток вверх (открытое окно ≥1s)
- доки (CONFIG.md/README/TESTING) + тесты (MetricsTest, BackoffTest)

src/commonTest/kotlin/pw/binom/llmproxy/BackoffTest.kt

@@ -4,11 +4,13 @@ import kotlinx.coroutines.test.runTest
 import kotlinx.datetime.Instant
 import kotlin.test.Test
 import kotlin.test.assertEquals
+import kotlin.test.assertFails
 import kotlin.test.assertFalse
 import kotlin.test.assertNull
 import kotlin.test.assertTrue
 import kotlin.time.Duration
 import kotlin.time.Duration.Companion.days
+import kotlin.time.Duration.Companion.hours
 import kotlin.time.Duration.Companion.milliseconds
 import kotlin.time.Duration.Companion.minutes
 import kotlin.time.Duration.Companion.seconds
@@ -53,6 +55,26 @@ class BackoffTest {
         assertEquals(90.seconds, Duration.parse("PT1M30S"))
     }
 
+    @Test
+    fun configDurationParsingWithYearsAndMonths() {
+        // kotlin.time не принимает Y/M — конфиг-парсер расширяет: 1Y≈365d, 1M≈30d
+        assertEquals(30.days, parseIsoDuration("P1M"))
+        assertEquals(365.days, parseIsoDuration("P1Y"))
+        assertEquals(455.days, parseIsoDuration("P1Y3M"))
+        assertEquals(7.days, parseIsoDuration("P1W"))
+        assertEquals(2.days + 3.hours + 15.minutes, parseIsoDuration("P2DT3H15M"))
+        // базовый ISO-8601 без Y/M — как kotlin.time
+        assertEquals(30.minutes, parseIsoDuration("PT30M"))
+    }
+
+    @Test
+    fun configDurationParsingRejectsGarbage() {
+        assertFails { parseIsoDuration("PT30") }
+        assertFails { parseIsoDuration("1d") }
+        assertFails { parseIsoDuration("P") }
+        assertFails { parseIsoDuration("") }
+    }
+
     @Test
     fun providerScopeSharedBetweenItsModels() = runTest {
         val p = ProviderConf(id = "p", url = "https://p", backoff = 5.minutes)