fix(admin): metrics endpoint not responding with Content-Type header

The prometheus metrics text format requires `Content-Type` header
for correct scraping which fails otherwise. Closes #4271.

Signed-off-by: Taimoor Zaeem <taimoorzaeem@gmail.com>

Author: taimoorzaeem

CHANGELOG.md

@@ -18,6 +18,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - Fix OpenAPI broken docs link by @taimoorzaeem in #4080
 - Fix OpenAPI specification incorrectly exposing GET methods for volatile functions by @joelonsql in #4174
 - Fix empty spread embeddings return unexpected SQL error by @taimoorzaeem in #3887
+- Fix `/metrics` endpoint not responding with `Content-Type` header by @taimoorzaeem in #4271
 
 ### Changed
 

docs/references/observability.rst

@@ -128,6 +128,11 @@ The ``metrics`` endpoint on the :ref:`admin_server` endpoint provides metrics in
 
   curl "http://localhost:3001/metrics"
 
+.. code-block:: http
+
+  HTTP/1.1 200 OK
+  Content-Type: text/plain; charset=utf-8
+
   # HELP pgrst_schema_cache_query_time_seconds The query time in seconds of the last schema cache load
   # TYPE pgrst_schema_cache_query_time_seconds gauge
   pgrst_schema_cache_query_time_seconds 1.5937927e-2

src/PostgREST/Admin.hs

@@ -16,6 +16,7 @@ import Network.Socket.ByteString
 
 import PostgREST.AppState    (AppState)
 import PostgREST.Config      (AppConfig (..))
+import PostgREST.MediaType   (MediaType (..), toContentType)
 import PostgREST.Metrics     (metricsToText)
 import PostgREST.Network     (resolveHost)
 import PostgREST.Observation (Observation (..))
@@ -58,7 +59,7 @@ admin appState req respond  = do
       respond $ Wai.responseLBS HTTP.status200 [] (maybe mempty JSON.encode sCache)
     ["metrics"] -> do
       mets <- metricsToText
-      respond $ Wai.responseLBS HTTP.status200 [] mets
+      respond $ Wai.responseLBS HTTP.status200 [toContentType MTTextPlain] mets -- Content-Type is required for prometheus compliance
     _ ->
       respond $ Wai.responseLBS HTTP.status404 [] mempty
 

test/io/test_io.py

@@ -1730,6 +1730,7 @@ def test_admin_metrics(defaultenv):
     with run(env=defaultenv, port=freeport()) as postgrest:
         response = postgrest.admin.get("/metrics")
         assert response.status_code == 200
+        assert response.headers["Content-Type"] == "text/plain; charset=utf-8"
         assert "pgrst_schema_cache_query_time_seconds" in response.text
         assert 'pgrst_schema_cache_loads_total{status="SUCCESS"}' in response.text
         assert "pgrst_db_pool_max" in response.text