fix: fix several problems after updating to the newest db

#24 introduced issues after porting to new DAO.
This change fixes the following:

- kong.log.err/warn instead of ERR/WARN
- usage of responses.lua instead of kong.response.exit
- unnecessary use of the consumer's schema in api.lua

From #26

kong/plugins/prometheus/api.lua

@@ -1,11 +1,20 @@
+local Schema = require("kong.db.schema")
 local prometheus = require "kong.plugins.prometheus.exporter"
 
-local consumers_schema = kong.db.consumers.schema
+-- schemas are used to parse parameters, for example: all params in a
+-- form-url-encoded field arrive to the server as strings. But if the provided
+-- schema has a field called `timeout` of type `number` then it will be transformed
+-- into a number before being passed down to the functions below.
+--
+-- On this particular case the Prometheus lib uses no schemas and the /metrics
+-- path accepts no parameters, but we still need to supply a schema in order to use the
+-- "new-db-style" admin API. So we generate an empty one on the fly.
+local empty_schema = Schema.new({ fields = {} })
 
 return {
 
   ["/metrics"] = {
-    schema = consumers_schema, -- not used, could be any schema
+    schema = empty_schema,
     methods = {
       GET = function()
         prometheus.collect()

kong/plugins/prometheus/exporter.lua

@@ -1,4 +1,3 @@
-local responses = require "kong.tools.responses"
 local find = string.find
 local select = select
 
@@ -12,7 +11,7 @@ local prometheus
 local function init()
   local shm = "prometheus_metrics"
   if not ngx.shared.prometheus_metrics then
-    kong.log.ERR("prometheus: ngx shared dict 'prometheus_metrics' not found")
+    kong.log.err("prometheus: ngx shared dict 'prometheus_metrics' not found")
     return
   end
 
@@ -41,7 +40,7 @@ end
 
 local function log(message)
   if not metrics then
-    kong.log.ERR("prometheus: can not log metrics because of an initialization "
+    kong.log.err("prometheus: can not log metrics because of an initialization "
                  .. "error, please make sure that you've declared "
                  .. "'prometheus_metrics' shared dict in your nginx template")
     return
@@ -82,15 +81,15 @@ end
 
 local function collect()
   if not prometheus or not metrics then
-    kong.log.ERR("prometheus: plugin is not initialized, please make sure ",
+    kong.log.err("prometheus: plugin is not initialized, please make sure ",
                  " 'prometheus_metrics' shared dict is present in nginx template")
-    return responses.send_HTTP_INTERNAL_SERVER_ERROR()
+    return kong.response.exit(500, { message = "An unexpected error occurred" })
   end
 
   local r = ngx.location.capture "/nginx_status"
 
   if r.status ~= 200 then
-    kong.log.WARN("prometheus: failed to retrieve /nginx_status ",
+    kong.log.warn("prometheus: failed to retrieve /nginx_status ",
                   "while processing /metrics endpoint")
 
   else
@@ -113,7 +112,7 @@ local function collect()
 
   else
     metrics.db_reachable:set(0)
-    kong.log.ERR("prometheus: failed to reach database while processing",
+    kong.log.err("prometheus: failed to reach database while processing",
                  "/metrics endpoint: ", err)
   end
 

kong/plugins/prometheus/handler.lua

@@ -29,7 +29,7 @@ function PrometheusHandler:log(conf) -- luacheck: ignore 212
   local message = basic_serializer.serialize(ngx)
   local ok, err = ngx.timer.at(0, log, message)
   if not ok then
-    kong.log.ERR("failed to create timer: ", err)
+    kong.log.err("failed to create timer: ", err)
   end
 end