Merge pull request #146 from palazzem/update-docs

[docs] be more explicit in docs

docs/GettingStarted.md

@@ -350,7 +350,7 @@ to trace requests to the home page:
 
         # add some attributes and metrics
         span.set_tag('http.method', request.request_method)
-        span.set_metric('posts.count', len(@posts))
+        span.set_tag('posts.count', @posts.length)
 
         # trace the template rendering
         tracer.trace('template.render') do

lib/ddtrace/span.rb

@@ -65,8 +65,8 @@ def get_tag(key)
       @meta[key]
     end
 
-    # Set the given key / value metric pair on the span. Keys must be string.
-    # Values must be floating point numbers.
+    # This method sets a tag with a floating point value for the given key. It acts
+    # like `set_tag()` and it simply add a tag without further processing.
     def set_metric(key, value)
       # enforce that the value is a floating point number
       value = Float(value)