Allow to pass custom span name provider

In current implementation the generated operation name
corresponds to the request's http method e.g. GET, POST etc. It's not
perfect when you look at UI and want to quickly understand what was
called. You need to dig into the trace to understand with what url it's
related.

This change introduces a capability to specify a custom span name
provider, and defaults to http method and host of the url.

lib/faraday/tracer.rb

@@ -1,8 +1,12 @@
+require 'uri'
 require 'faraday'
 require 'opentracing'
 
 module Faraday
   class Tracer < Faraday::Middleware
+    MethodOnly = ->(env) { env[:method].to_s.upcase }
+    MethodWithHost = ->(env) { "#{env[:method].to_s.upcase} #{URI(env[:url].to_s).host}".strip }
+
     # Create a new Faraday Tracer middleware.
     #
     # @param app The faraday application/middlewares stack.
@@ -13,15 +17,16 @@ class Tracer < Faraday::Middleware
     #        is called.
     # @param errors [Array<Class>] An array of error classes to be captured by the tracer
     #        as errors. Errors are **not** muted by the middleware.
-    def initialize(app, span: nil, tracer: OpenTracing.global_tracer, errors: [StandardError])
+    def initialize(app, span: nil, tracer: OpenTracing.global_tracer, errors: [StandardError], span_name_provider: MethodWithHost)
       super(app)
       @tracer = tracer
       @parent_span = span
       @errors = errors
+      @span_name_provider = span_name_provider
     end
 
     def call(env)
-      span = @tracer.start_span(env[:method].to_s.upcase,
+      span = @tracer.start_span(@span_name_provider.call(env),
         child_of: @parent_span.respond_to?(:call) ? @parent_span.call : @parent_span,
         tags: {
           'component' => 'faraday',

spec/faraday/tracer_spec.rb

@@ -7,6 +7,7 @@
   it 'uses upcase HTTP method as span operation name' do
     call(method: :post)
     span = tracer.finished_spans.first
+    puts span.operation_name
     expect(span.operation_name).to eq('POST')
   end