chore(dbm): add injection mode environment variable (#4306)

## Description

Adds `DD_TRACE_SQL_COMMENT_INJECTION_MODE` environment variable 

## Checklist
- [ ] Title must conform to [conventional commit](https://github.com/conventional-changelog/commitlint/tree/master/%40commitlint/config-conventional).
- [ ] Add additional sections for `feat` and `fix` pull requests.
- [ ] Ensure tests are passing for affected code.
- [ ] [Library documentation](https://github.com/DataDog/dd-trace-py/tree/1.x/docs) and/or [Datadog's documentation site](https://github.com/DataDog/documentation/) is updated. Link to doc PR in description.





## Motivation


## Design 


## Testing strategy




## Relevant issue(s)


## Testing strategy




## Reviewer Checklist
- [ ] Title is accurate.
- [ ] Description motivates each change.
- [ ] No unnecessary changes were introduced in this PR.
- [ ] PR cannot be broken up into smaller PRs.
- [ ] Avoid breaking [API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces) changes unless absolutely necessary.
- [ ] Tests provided or description of manual testing performed is included in the code or PR.
- [ ] Release note has been added for fixes and features, or else `changelog/no-changelog` label added.
- [ ] All relevant GitHub issues are correctly linked.
- [ ] Backports are identified and tagged with Mergifyio.
- [ ] Add to milestone.

Author: mabdinur

ddtrace/settings/database_monitoring.py

@@ -0,0 +1,17 @@
+from envier import En
+from envier import validators
+
+
+class DatabaseMonitoringConfig(En):
+    __prefix__ = "dd_trace"
+
+    injection_mode = En.v(
+        str,
+        "sql_comment_injection_mode",
+        default="disabled",
+        help="Valid Injection Modes: disabled, service, and full",
+        validator=validators.choice(["disabled", "full", "service"]),
+    )
+
+
+config = DatabaseMonitoringConfig()

tests/internal/test_database_monitoring.py

@@ -0,0 +1,29 @@
+import pytest
+
+from ddtrace.settings import database_monitoring
+from tests.utils import override_env
+
+
+def test_injection_mode_configuration():
+    # Ensure Database Monitoring support is disabled by default
+    # TODO: Enable DBM support by default
+    assert database_monitoring.config.injection_mode == "disabled"
+
+    # Ensure service is a valid injection mode
+    with override_env(dict(DD_TRACE_SQL_COMMENT_INJECTION_MODE="service")):
+        config = database_monitoring.DatabaseMonitoringConfig()
+        assert config.injection_mode == "service"
+
+    # Ensure full is a valid injection mode
+    with override_env(dict(DD_TRACE_SQL_COMMENT_INJECTION_MODE="full")):
+        config = database_monitoring.DatabaseMonitoringConfig()
+        assert config.injection_mode == "full"
+
+    # Ensure an invalid injection mode raises a ValueError
+    with override_env(dict(DD_TRACE_SQL_COMMENT_INJECTION_MODE="notaninjectionmode")):
+        with pytest.raises(ValueError) as excinfo:
+            database_monitoring.DatabaseMonitoringConfig()
+    assert (
+        excinfo.value.args[0] == "Invalid value for environment variable DD_TRACE_SQL_COMMENT_INJECTION_MODE: "
+        "value must be one of ['disabled', 'full', 'service']"
+    )