Introduce resource_name_override option

README.md

@@ -93,6 +93,48 @@ defmodule ExampleImpl do
   use CarReq, datadog_service_name: :dont_put_me_in_a_box
 end
 ```
+
+### Resource Name Overrides
+
+In most cases, your application's instrumentation modules (the ones that call `:telemetry.attach/4` on Telemetry events executed by CarReq) will take the metadata provided by CarReq's Telemetry events and form a resource name that may look something like:
+
+```
+get some_partner.api.com/product/4123/details
+```
+
+In some cases, you may want specific requests or clients to override the default resource name provided by your application's instrumentation module. To do so, you can pass along a `:resource_name_override` option at the client level or on a per-request
+basis. The value of `:resource_name_override` must be a 1-arity function that takes a string (the default resource_name provided by your
+instrumentation) and returns a new string (the overridden resource_name). Note that when passing this option at the client-level, you
+must pass the option as an external function capture, i.e., `&Module.function/1`. An example would be:
+
+```elixir
+defmodule MyResourceOverrideClient do
+  use CarReq, resource_name_override: &MyResourceOverrideClient.override_function/1
+
+  @doc """
+  Replaces any consecutive instances of numbers in a string with the
+  placeholder `{guid}`. For example
+
+  MyResourceOverrideClient.override_function("get partner.api.com/product/1234/details")
+  # => "get partner.api.com/product/{guid}/details"
+  """
+  def override_function(resource_name) do
+    String.replace(resource_name, ~r|\d+|, "{guid}")
+  end
+end
+```
+
+You can also provide an override function on a per-request basis as follows:
+
+```elixir
+MyResourceOverrideClient.request(
+  method: :get,
+  url: url,
+  resource_name_override: fn resource_name -> 
+   String.replace(resource_name, ~r|\d+|, "{guid}")
+  end
+```
+
 ## Options
 
   # adapter

lib/car_req.ex

@@ -72,6 +72,9 @@ defmodule CarReq do
     ],
     fuse_melt_func: [
       type: {:fun, 1}
+    ],
+    resource_name_override: [
+      type: {:fun, 1}
     ]
   ]
 
@@ -183,7 +186,8 @@ defmodule CarReq do
     Req.new()
     |> Req.Request.register_options([
       :datadog_service_name,
-      :implementing_module
+      :implementing_module,
+      :resource_name_override
     ])
     |> LogStep.attach()
     |> CarReq.attach_circuit_breaker(options)
@@ -268,12 +272,16 @@ defmodule CarReq do
         |> Keyword.merge(request_options)
       end
 
+      resource_name_override = Keyword.get(opts, :resource_name_override)
+
       defp telemetry_metadata(request_options) do
         %{
           datadog_service_name:
             Keyword.get(request_options, :datadog_service_name, @datadog_service_name),
           url: Keyword.get(request_options, :url),
           method: Keyword.get(request_options, :method),
+          resource_name_override:
+            Keyword.get(request_options, :resource_name_override, unquote(resource_name_override)),
           query_params: Keyword.get(request_options, :params)
         }
       end

test/car_req_test.exs

@@ -756,6 +756,53 @@ defmodule CarReqTest do
                       %{datadog_service_name: :guu_car_caz}}
     end
 
+    test "allows client to set a resource_name_override rule" do
+      defmodule ResourceNameOverrideClient do
+        use CarReq,
+          resource_name_override: &ResourceNameOverrideClient.resource_name_override/1
+
+        def resource_name_ovveride(resource) do
+          String.replace(resource, ~r|\d+|, "{guid}")
+        end
+      end
+
+      ResourceNameOverrideClient.request(method: :get, url: "/200", adapter: &Adapter.success/1)
+
+      assert_receive {:event, [:http_car_req, :request, :start], _,
+                      %{resource_name_override: override}}
+
+      assert is_function(override, 1)
+    end
+
+    test "allows resource name override option to be set per-request on the client" do
+      defmodule ResourceNameOverrideRuntimeClient do
+        use CarReq,
+          resource_name_override: &ResourceNameOverrideRuntimeClient.global_override/1
+
+        def request_override(resource_name) do
+          String.replace(resource_name, ~r|\d+|, "{guid}")
+        end
+
+        def global_override(resource_name) do
+          String.replace(resource_name, "/", "<slash>")
+        end
+      end
+
+      url = "/employees/1234/details"
+
+      ResourceNameOverrideRuntimeClient.request(
+        method: :get,
+        url: url,
+        adapter: &Adapter.success/1,
+        resource_name_override: &ResourceNameOverrideRuntimeClient.request_override/1
+      )
+
+      assert_receive {:event, [:http_car_req, :request, :start], _,
+                      %{resource_name_override: override_fun}}
+
+      assert override_fun.(url) == ResourceNameOverrideRuntimeClient.request_override(url)
+    end
+
     test "determines service name for external namespaced clients" do
       defmodule Test.Foo.Bar.External.Service.ServiceNameClient do
         use CarReq