feat(datadog): response_status_class tag to metrics

Having a separate tag for the HTTP response code class allows useful
grouping in Datadog by e.g. successful requests (2xx), client errors
(4xx), and server errors (5xx). The existing `response_status` already
essentially includes that information, but because of how Datadog works,
one would need to list out each possible value between 400 and 431 to
group all client errors, for example. Having a separate tag for the
class greatly simplifies this.

Generally it may be problematic to add new tags by default without an
opt-in configuration option, because Datadog charges based on [custom
metrics count][1] and additional tags can increase that count. However,
the additional tag is safe to add here, because it is guaranteed not to
increase the custom metric count. That is because the new tag is based
on the already included `response_status` value, which is more granular
than the new `response_status_class` value, so the number of unique tag
combinations will not change.

[1]: https://docs.datadoghq.com/metrics/custom_metrics/

Author: deiwin

apisix/plugins/datadog.lua

@@ -25,6 +25,7 @@ local format = string.format
 local concat = table.concat
 local tostring = tostring
 local pairs = pairs
+local floor = math.floor
 
 local plugin_name = "datadog"
 local defaults = {
@@ -118,6 +119,7 @@ local function generate_tag(entry, const_tags)
     end
     if entry.response.status then
         core.table.insert(tags, "response_status:" .. entry.response.status)
+        core.table.insert(tags, "response_status_class:" .. floor(entry.response.status / 100) .. "xx")
     end
     if entry.scheme ~= "" then
         core.table.insert(tags, "scheme:" .. entry.scheme)

docs/en/latest/plugins/datadog.md

@@ -116,7 +116,8 @@ The metrics will be sent to the DogStatsD agent with the following tags:
 - `service_name`: If a Route has been created with an abstracted Service, the Service name/ID based on the attribute `prefer_name`.
 - `consumer`: If the Route is linked to a Consumer, the username will be added as a tag.
 - `balancer_ip`: IP address of the Upstream balancer that processed the current request.
-- `response_status`: HTTP response status code.
+- `response_status`: HTTP response status code. E.g. "200", "404", "503".
+- `response_status_class`: HTTP response status code class. E.g. "2xx", "4xx", "5xx".
 - `scheme`: Request scheme such as HTTP, gRPC, and gRPCs.
 - `path`: The HTTP path pattern. Only available if the attribute `include_path` is set to true.
 - `method`: The HTTP method. Only available if the attribute `include_method` is set to true.