[DBMON-6781] Implement agent_hostname in DatabaseCheck (#24243)

* Implement agent_hostname in DatabaseCheck

* Add changelog

Author: eric-weaver

datadog_checks_base/changelog.d/24243.added

@@ -0,0 +1 @@
+Implement the ``agent_hostname`` property on the ``DatabaseCheck`` base class.

datadog_checks_base/datadog_checks/base/checks/db.py

@@ -4,6 +4,7 @@
 
 from abc import abstractmethod
 
+from datadog_checks.base.agent import datadog_agent
 from datadog_checks.base.utils.db.utils import TagManager
 
 from . import AgentCheck
@@ -12,6 +13,7 @@
 class DatabaseCheck(AgentCheck):
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
+        self._agent_hostname = None
         self.tag_manager = TagManager()
 
     def database_monitoring_query_sample(self, raw_event: str):
@@ -39,6 +41,12 @@ def reported_hostname(self) -> str | None:
     def database_identifier(self) -> str:
         pass
 
+    @property
+    def agent_hostname(self) -> str:
+        if self._agent_hostname is None:
+            self._agent_hostname = datadog_agent.get_hostname()
+        return self._agent_hostname
+
     @property
     def dbms(self) -> str:
         return self.__class__.__name__.lower()

datadog_checks_base/tests/base/checks/test_database_check.py

@@ -0,0 +1,38 @@
+# (C) Datadog, Inc. 2026-present
+# All rights reserved
+# Licensed under a 3-clause BSD style license (see LICENSE)
+from unittest import mock
+
+from datadog_checks.base.checks.db import DatabaseCheck
+from datadog_checks.base.stubs.datadog_agent import datadog_agent
+
+
+class FakeDatabaseCheck(DatabaseCheck):
+    @property
+    def reported_hostname(self):
+        return None
+
+    @property
+    def database_identifier(self):
+        return "test-db"
+
+    @property
+    def dbms_version(self):
+        return "1.0.0"
+
+    @property
+    def tags(self):
+        return []
+
+    @property
+    def cloud_metadata(self):
+        return {}
+
+
+def test_agent_hostname_resolves_once_and_caches():
+    check = FakeDatabaseCheck("test", {}, [{}])
+    # The hostname comes from an FFI call, so it should only be resolved once and cached.
+    with mock.patch.object(datadog_agent, "get_hostname", return_value="my-agent-host") as get_hostname:
+        assert check.agent_hostname == "my-agent-host"
+        assert check.agent_hostname == "my-agent-host"
+        assert get_hostname.call_count == 1