Add minimal prometheus exporter support

Prometheus https://prometheus.io/ is one of the popular monitoring
systems. It pulls service's endpoint to get metrics in a simple
text format https://prometheus.io/docs/instrumenting/exposition_formats/

kvmd/apps/kvmd/http.py

@@ -147,6 +147,23 @@ def make_json_exception(err: Exception, status: Optional[int]=None) -> aiohttp.w
     }, status=status)
 
 
+def make_text_response(
+    result: str,
+    status: int=200,
+    set_cookies: Optional[Dict[str, str]]=None,
+) -> aiohttp.web.Response:
+
+    response = aiohttp.web.Response(
+        text=result,
+        status=status,
+        content_type="text/plain",
+    )
+    if set_cookies:
+        for (key, value) in set_cookies.items():
+            response.set_cookie(key, value)
+    return response
+
+
 # =====
 async def get_multipart_field(reader: aiohttp.MultipartReader, name: str) -> aiohttp.BodyPartReader:
     field = await reader.next()