chore(dbm): use peer.service precursor when enabled [backport 1.19] (#7034)

Backport 466fd2a from #6962 to 1.19.

DBM adds comments to database commands for the purposes of context
propagation. One of the pieces of information in the comments is the
service name of the external database and is used for generating metrics
(i.e. # of writes per second to external database).

Prior to this change, DBM relied on the service name being set to the
value of span.service. In auto-instrumented services, this value
typically belongs to the fake span generated with the destination
database name, such as "postgres". This caused issues when peer.service
is enabled, since the fake span no longer existed.

After this change, DBM will use the precursor for peer.service (if
available) when peer service defaults are enabled.

## Checklist

- [x] Change(s) are motivated and described in the PR description.
- [x] Testing strategy is described if automated tests are not included
in the PR.
- [x] Risk is outlined (performance impact, potential for breakage,
maintainability, etc).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
are followed. If no release note is required, add label
`changelog/no-changelog`.
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/)).
- [x] Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist

- [x] Title is accurate.
- [x] No unnecessary changes are introduced.
- [x] Description motivates each change.
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
- [x] Testing strategy adequately addresses listed risk(s).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] Release note makes sense to a user of the library.
- [x] Reviewer has explicitly acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment.
- [x] Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)
- [x] If this PR touches code that signs or publishes builds or
packages, or handles credentials of any kind, I've requested a review
from `@DataDog/security-design-and-guidance`.
- [x] This PR doesn't touch any of that.

Co-authored-by: Teague Bick <[email protected]>

Author: github-actions[bot]

ddtrace/propagation/_database_monitoring.py

@@ -2,8 +2,10 @@
 from typing import Union  # noqa
 
 from ddtrace.internal.logger import get_logger
+from ddtrace.settings.peer_service import PeerServiceConfig
 from ddtrace.vendor.sqlcommenter import generate_sql_comment as _generate_sql_comment
 
+from ..internal import compat
 from ..internal.utils import get_argument_value
 from ..internal.utils import set_argument_value
 from ..settings import _config as dd_config
@@ -71,11 +73,17 @@ def _get_dbm_comment(self, db_span):
             return None
 
         # set the following tags if DBM injection mode is full or service
+        peer_service_enabled = PeerServiceConfig().set_defaults_enabled
+        service_name_key = db_span.service
+        if peer_service_enabled:
+            db_name = db_span.get_tags().get("db.name")
+            service_name_key = compat.ensure_str(db_name) if db_name else db_span.service
+
         dbm_tags = {
             DBM_PARENT_SERVICE_NAME_KEY: dd_config.service,
             DBM_ENVIRONMENT_KEY: dd_config.env,
             DBM_VERSION_KEY: dd_config.version,
-            DBM_DATABASE_SERVICE_NAME_KEY: db_span.service,
+            DBM_DATABASE_SERVICE_NAME_KEY: service_name_key,
         }
 
         if dbm_config.propagation_mode == "full":

releasenotes/notes/dbm-fix-for-peer-service-ea46572a0daa08ec.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    dbm: this fixes an issue with attributing metrics to incorrect services when peer.service is enabled

tests/internal/test_database_monitoring.py

@@ -119,6 +119,45 @@ def test_dbm_propagation_full_mode():
     assert dbspan.get_tag(_database_monitoring.DBM_TRACE_INJECTED_TAG) == "true"
 
 
+@pytest.mark.subprocess(
+    env=dict(
+        DD_DBM_PROPAGATION_MODE="full",
+        DD_SERVICE="orders-app",
+        DD_ENV="staging",
+        DD_VERSION="v7343437-d7ac743",
+        DD_TRACE_PEER_SERVICE_DEFAULTS_ENABLED="True",
+    )
+)
+def test_dbm_dddbs_peer_service_enabled():
+    from ddtrace import tracer
+    from ddtrace.propagation import _database_monitoring
+
+    dbspan_no_service = tracer.trace("dbname")
+
+    # when dbm propagation mode is full sql comments should be generated with dbm tags and traceparent keys
+    dbm_popagator = _database_monitoring._DBM_Propagator(0, "procedure")
+    sqlcomment = dbm_popagator._get_dbm_comment(dbspan_no_service)
+    # assert tags sqlcomment contains the correct value
+    assert (
+        sqlcomment
+        == "/*dddbs='orders-app',dde='staging',ddps='orders-app',ddpv='v7343437-d7ac743',traceparent='%s'*/ "
+        % (dbspan_no_service.context._traceparent,)
+    ), sqlcomment
+
+    dbspan_with_peer_service = tracer.trace("dbname")
+    dbspan_with_peer_service.set_tag_str("db.name", "db-name-test")
+
+    # when dbm propagation mode is full sql comments should be generated with dbm tags and traceparent keys
+    dbm_popagator = _database_monitoring._DBM_Propagator(0, "procedure")
+    sqlcomment = dbm_popagator._get_dbm_comment(dbspan_with_peer_service)
+    # assert tags sqlcomment contains the correct value
+    assert (
+        sqlcomment
+        == "/*dddbs='db-name-test',dde='staging',ddps='orders-app',ddpv='v7343437-d7ac743',traceparent='%s'*/ "
+        % (dbspan_with_peer_service.context._traceparent,)
+    ), sqlcomment
+
+
 def test_default_sql_injector(caplog):
     # test sql injection with unicode str
     dbm_comment = "/*dddbs='orders-db'*/ "