Fix connection pool timeouts unit (#7404)

And add a test that actually verifies it

instrumentation-api-semconv/src/main/java/io/opentelemetry/instrumentation/api/metrics/db/DbConnectionPoolMetrics.java

@@ -108,7 +108,7 @@ public final class DbConnectionPoolMetrics {
   public LongCounter connectionTimeouts() {
     return meter
         .counterBuilder("db.client.connections.timeouts")
-        .setUnit("timeouts")
+        .setUnit("{timeouts}")
         .setDescription(
             "The number of connection timeouts that have occurred trying to obtain a connection from the pool.")
         .build();

instrumentation/hikaricp-3.0/testing/src/main/java/io/opentelemetry/instrumentation/hikaricp/AbstractHikariInstrumentationTest.java

@@ -5,6 +5,8 @@
 
 package io.opentelemetry.instrumentation.hikaricp;
 
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.catchException;
 import static org.mockito.ArgumentMatchers.anyLong;
 import static org.mockito.Mockito.atLeastOnce;
 import static org.mockito.Mockito.verify;
@@ -53,6 +55,8 @@ public abstract class AbstractHikariInstrumentationTest {
     hikariDataSource.setDataSource(dataSourceMock);
     configure(hikariDataSource, null);
 
+    cleanup.deferCleanup(hikariDataSource);
+
     // when
     Connection hikariConnection = hikariDataSource.getConnection();
     TimeUnit.MILLISECONDS.sleep(100);
@@ -126,4 +130,38 @@ public abstract class AbstractHikariInstrumentationTest {
     verify(userMetricsMock, atLeastOnce()).recordConnectionAcquiredNanos(anyLong());
     verify(userMetricsMock, atLeastOnce()).recordConnectionUsageMillis(anyLong());
   }
+
+  @Test
+  void shouldReportTimeouts() throws SQLException {
+    // given
+    when(dataSourceMock.getConnection())
+        .then(
+            invocation -> {
+              TimeUnit.MILLISECONDS.sleep(2_000);
+              throw new SQLException("timed out!");
+            });
+
+    HikariDataSource hikariDataSource = new HikariDataSource();
+    hikariDataSource.setPoolName("timingOutPool");
+    hikariDataSource.setDataSource(dataSourceMock);
+    hikariDataSource.setConnectionTimeout(250 /* millis */);
+    // start the pool without initializing connections
+    hikariDataSource.setInitializationFailTimeout(-1);
+    configure(hikariDataSource, null);
+
+    cleanup.deferCleanup(hikariDataSource);
+
+    // when
+    Exception thrown = catchException(hikariDataSource::getConnection);
+
+    // then
+    assertThat(thrown).isNotNull();
+
+    DbConnectionPoolMetricsAssertions.create(
+            testing(), "io.opentelemetry.hikaricp-3.0", "timingOutPool")
+        .disableMaxIdleConnections()
+        // the connection is not even acquired
+        .disableUseTime()
+        .assertConnectionPoolEmitsMetrics();
+  }
 }