fix: avoid recording duplicate metrics

Problem:
All metrics are recorded by incrementing counters in a shared dictionary.
As the number of metrics increase, so does the contention among Nginx
worker process to increase those counters increases leading to higher
CPU utilization(#2) and potentially will result in a drop in peak RPS.
This also affects the collection phase of metrics where the shared
dictionary needs to be locked for a longer time.

Solution:
Since half of the metrics being recorded are duplicate anyways, those
metrics can be dropped. This doesn't result in any loss functionally
since these metrics can be easily in Prometheus.

From #8

kong/plugins/prometheus/exporter.lua

@@ -29,16 +29,6 @@ local function init()
                                          {"state"})
   metrics.db_reachable = prometheus:gauge("datastore_reachable",
                                           "Datastore reachable from Kong, 0 is unreachable")
-  metrics.status_total = prometheus:counter("http_status_total",
-                                            "HTTP status codes aggreggated across all services in Kong",
-                                            {"code"})
-  metrics.latency_total = prometheus:histogram("latency_total",
-                                               "Latency added by Kong, total request time and upstream latency aggreggated across all services in Kong",
-                                               {"type"},
-                                               DEFAULT_BUCKETS) -- TODO make this configurable
-  metrics.bandwidth_total = prometheus:counter("bandwidth_total",
-                                               "Total bandwidth in bytes for all proxied requests in Kong",
-                                               {"type"})
 
   -- per service
   metrics.status = prometheus:counter("http_status",
@@ -67,36 +57,30 @@ local function log(message)
   service_name = service_name or ""
 
   metrics.status:inc(1, { message.response.status, service_name })
-  metrics.status_total:inc(1, { message.response.status })
 
   local request_size = tonumber(message.request.size)
   if request_size and request_size > 0 then
     metrics.bandwidth:inc(request_size, { "ingress", service_name })
-    metrics.bandwidth_total:inc(request_size, { "ingress" })
   end
 
   local response_size = tonumber(message.response.size)
   if response_size and response_size > 0 then
     metrics.bandwidth:inc(response_size, { "egress", service_name })
-    metrics.bandwidth_total:inc(response_size, { "egress" })
   end
 
   local request_latency = tonumber(message.latencies.request)
   if request_latency and request_latency >= 0 then
     metrics.latency:observe(request_latency, { "request", service_name })
-    metrics.latency_total:observe(request_latency, { "request" })
   end
 
   local upstream_latency = tonumber(message.latencies.proxy)
   if upstream_latency ~= nil and upstream_latency >= 0 then
     metrics.latency:observe(upstream_latency, {"upstream", service_name })
-    metrics.latency_total:observe(upstream_latency, { "upstream" })
   end
 
   local kong_proxy_latency = tonumber(message.latencies.kong)
   if kong_proxy_latency ~= nil and kong_proxy_latency >= 0 then
     metrics.latency:observe(kong_proxy_latency, { "kong", service_name })
-    metrics.latency_total:observe(kong_proxy_latency, { "kong" })
   end
 end
 

spec/02-access_spec.lua

@@ -57,7 +57,6 @@ describe("Plugin: prometheus (access)", function()
       path    = "/metrics",
     })
     local body = assert.res_status(200, res)
-    assert.matches('kong_http_status_total{code="200"} 1', body, nil, true)
     assert.matches('kong_http_status{code="200",service="mock-service"} 1', body, nil, true)
 
     ngx.sleep(1)
@@ -75,7 +74,6 @@ describe("Plugin: prometheus (access)", function()
       path    = "/metrics",
     })
     local body = assert.res_status(200, res)
-    assert.matches('kong_http_status_total{code="400"} 1', body, nil, true)
     assert.matches('kong_http_status{code="400",service="mock-service"} 1', body, nil, true)
   end)
 end)

spec/03-custom-serve_spec.lua

@@ -55,7 +55,6 @@ describe("Plugin: prometheus (custom server)",function()
     	  path    = "/metrics",
     	})
     	local body = assert.res_status(200, res)
-    	assert.matches('kong_http_status_total{code="200"} 1', body, nil, true)
     	assert.matches('kong_http_status{code="200",service="mock-service"} 1', body, nil, true)
     end)
   end)