:bug: do not override Server-Timing if it exists

[codingjoe]

The middleware just overrides the Server-Timing header if it exists. It should simply amend measurements to be non-destructive.

[tim-schilling]

To reproduce:

# tests/views.py
def server_timing(request):
    response = execute_sql(request)
    response.headers["Server-Timing"] = 'existing_key;dur=100;desc="Details"'
    return response

# tests/urls.py
urlpatterns = [
    # ...
    path("server-timing/", views.server_timing),
]

# tests/test_integration.py
class DebugToolbarIntegrationTestCase:
    def test_server_timing_headers_with_existing_value(self):
        response = self.client.get("/server-timing/")
        server_timing = response["Server-Timing"]
        expected_partials = [
            'existing_key;dur=100;desc="Details"',
            r'TimerPanel_utime;dur=(\d)*(\.(\d)*)?;desc="User CPU time", ',
            r'TimerPanel_stime;dur=(\d)*(\.(\d)*)?;desc="System CPU time", ',
            r'TimerPanel_total;dur=(\d)*(\.(\d)*)?;desc="Total CPU time", ',
            r'TimerPanel_total_time;dur=(\d)*(\.(\d)*)?;desc="Elapsed time", ',
            r'SQLPanel_sql_time;dur=(\d)*(\.(\d)*)?;desc="SQL 1 queries", ',
            r'CachePanel_total_time;dur=0;desc="Cache 0 Calls"',
        ]
        for expected in expected_partials:
            self.assertTrue(re.compile(expected).search(server_timing))

The run tox -e <env> or make test

[codingjoe]

Holy moly, @tim-schilling & @JohananOppongAmoateng that was quick! Especially considering my poorly written one-liner ticket. Thanks! I appreciate the swift effort!

… and here I was just vibing some ideas for a new health-check release codingjoe/django-health-check#571