fix(kork-sql): Use class variables for hikari pool metrics (#1081)

The SQL pool gauge metrics are disappearing due to the adhoc gauge
objects being garbage collected. This change fixes the pool metrics to be
class variables instead and hence avoid being garbage collected.
The micrometer docs also alludes to this issue:
https://micrometer.io/docs/concepts#_why_is_my_gauge_reporting_nan_or_disappearing

kork-sql/src/main/kotlin/com/netflix/spinnaker/kork/sql/telemetry/HikariSpectatorMetricsTracker.kt

@@ -33,19 +33,19 @@ class HikariSpectatorMetricsTracker(
  private val connectionUsageId = registry.createId("sql.pool.$poolName.connectionUsageTiming")
  private val connectionTimeoutId = registry.createId("sql.pool.$poolName.connectionTimeout")
 
- private val idleConnectionsId = registry.createId("sql.pool.$poolName.idle")
- private val activeConnectionsId = registry.createId("sql.pool.$poolName.active")
- private val totalConnectionsId = registry.createId("sql.pool.$poolName.total")
- private val blockedThreadsId = registry.createId("sql.pool.$poolName.blocked")
+ private val idleConnectionsGauge = registry.gauge("sql.pool.$poolName.idle")
+ private val activeConnectionsGauge = registry.gauge("sql.pool.$poolName.active")
+ private val totalConnectionsGauge = registry.gauge("sql.pool.$poolName.total")
+ private val blockedThreadsGauge = registry.gauge("sql.pool.$poolName.blocked")
 
  /**
  * Record the individual pool's statistics.
  */
  fun recordPoolStats() {
- registry.gauge(idleConnectionsId).set(poolStats.idleConnections.toDouble())
- registry.gauge(activeConnectionsId).set(poolStats.activeConnections.toDouble())
- registry.gauge(totalConnectionsId).set(poolStats.totalConnections.toDouble())
- registry.gauge(blockedThreadsId).set(poolStats.pendingThreads.toDouble())
+ idleConnectionsGauge.set(poolStats.idleConnections.toDouble())
+ activeConnectionsGauge.set(poolStats.activeConnections.toDouble())
+ totalConnectionsGauge.set(poolStats.totalConnections.toDouble())
+ blockedThreadsGauge.set(poolStats.pendingThreads.toDouble())
  }
 
  override fun recordConnectionAcquiredNanos(elapsedAcquiredNanos: Long) {