Add support for custom logger via configuration (#225)

Fixes #223.

---------

Co-authored-by: Carmine Paolino <carmine@paolino.me>

Author: tristandunn

docs/configuration.md

@@ -86,6 +86,7 @@ RubyLLM.configure do |config|
   # config.http_proxy = "socks5://proxy.company.com:1080"        # SOCKS5 proxy
 
   # --- Logging Settings ---
+  # config.logger = Rails.logger # When set the file and level settings are not used.
   config.log_file = '/logs/ruby_llm.log'
   config.log_level = :debug # debug level can also be set to debug by setting RUBYLLM_DEBUG envar to true
   config.log_assume_model_exists = false # Silence "Assuming model exists for provider" warning
@@ -158,13 +159,16 @@ Fine-tune how RubyLLM handles HTTP connections and retries.
 Adjust these based on network conditions and provider reliability.
 
 ## Logging Settings
-RubyLLM provides flexible logging configuration to help you monitor and debug API interactions. You can configure both the log file location and the logging level.
+RubyLLM provides flexible logging configuration to help you monitor and debug API interactions. You can configure both the log file location and the logging level, or set a custom logger.
 
 ```ruby
 RubyLLM.configure do |config|
   # --- Logging Settings ---
   config.log_file = '/logs/ruby_llm.log'  # Path to log file (default: nil, logs to STDOUT)
   config.log_level = :debug  # Log level (:debug, :info, :warn)
+
+  # --- Custom Logger ---
+  config.logger = Rails.logger
 end
 ```
 
@@ -181,6 +185,14 @@ end
 
 You can also set the debug level by setting the `RUBYLLM_DEBUG` environment variable to `true`.
 
+### Custom Logger
+
+* `config.logger`: Specifies a custom `Logger` for where logs should be written.
+
+{: .note }
+If you set a customer logger the `config.log_file` and `config.log_level`
+settings are not used.
+
 ## Scoped Configuration with Contexts
 
 While `RubyLLM.configure` sets global defaults, `RubyLLM.context` allows you to create temporary, isolated configuration scopes for specific API calls. This is ideal for situations requiring different keys, endpoints, or timeouts temporarily without affecting the rest of the application.

lib/ruby_llm.rb

@@ -67,7 +67,7 @@ def config
     end
 
     def logger
-      @logger ||= Logger.new(
+      @logger ||= config.logger || Logger.new(
         config.log_file,
         progname: 'RubyLLM',
         level: config.log_level

lib/ruby_llm/configuration.rb

@@ -36,6 +36,7 @@ class Configuration
                   :retry_interval_randomness,
                   :http_proxy,
                   # Logging configuration
+                  :logger,
                   :log_file,
                   :log_level,
                   :log_assume_model_exists

spec/ruby_llm_spec.rb

@@ -0,0 +1,50 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe RubyLLM do
+  describe '.logger' do
+    let(:logger) { instance_double(Logger) }
+    let(:log_file) { double }
+    let(:log_level) { double }
+
+    before do
+      described_class.instance_variable_set(:@config, nil)
+      described_class.instance_variable_set(:@logger, nil)
+    end
+
+    after do
+      described_class.instance_variable_set(:@config, nil)
+      described_class.instance_variable_set(:@logger, nil)
+    end
+
+    context 'with configuration options' do
+      before do
+        described_class.configure do |config|
+          config.log_file = log_file
+          config.log_level = log_level
+        end
+      end
+
+      it 'returns a default Logger' do
+        allow(Logger).to receive(:new).with(log_file, progname: 'RubyLLM', level: log_level).and_return(logger)
+
+        expect(described_class.logger).to eq(logger)
+      end
+    end
+
+    context 'with a custom logger' do
+      before do
+        described_class.configure do |config|
+          config.logger = logger
+          config.log_file = log_file
+          config.log_level = log_level
+        end
+      end
+
+      it 'returns a the custom Logger' do
+        expect(described_class.logger).to eq(logger)
+      end
+    end
+  end
+end