Expose default_url in config

No reason to keep this value hidden, and we want to reuse it for tests.

src/client/config.ml

@@ -36,6 +36,8 @@ let pp out (self : t) : unit =
     debug self_trace url_traces url_metrics url_logs ppheaders headers ppiopt
     batch_traces ppiopt batch_metrics ppiopt batch_logs batch_timeout_ms
 
+let default_url = "http://localhost:4318"
+
 type 'k make =
   ?debug:bool ->
   ?url:string ->
@@ -73,8 +75,6 @@ module Env () : ENV = struct
 
   let set_debug b = debug_ := b
 
-  let default_url = "http://localhost:4318"
-
   let make_get_from_env env_name =
     let value = ref None in
     fun () ->

src/client/config.mli

@@ -38,6 +38,9 @@ type t = private {
     To build one, use {!make} below. This might be extended with more fields in
     the future. *)
 
+val default_url : string
+(** The default base URL for the config. *)
+
 val pp : Format.formatter -> t -> unit
 
 type 'k make =
@@ -59,8 +62,8 @@ type 'k make =
 
     @param url
       base url used to construct per-signal urls. Per-signal url options take
-      precedence over this base url. Default is "http://localhost:4318", or
-      "OTEL_EXPORTER_OTLP_ENDPOINT" if set.
+      precedence over this base url. If not provided, this defaults to
+      "OTEL_EXPORTER_OTLP_ENDPOINT" if set, or if not {!default_url}.
 
     Example of constructed per-signal urls with the base url
     http://localhost:4318