Introduce setTag method for a Span interface and get rid of setTags

Having setTags instead of setTag means that you would have to be aware
of tags that are set in order to add aditional tag to a span. But that is
not the case if we expose an interface to set just a single tag.

All other opentracing libraries also have setTag exposed instead of
setTags so this helps to have more consistency between libraries.

Closes #51

src/OpenTracing/NoopSpan.php

@@ -42,7 +42,7 @@ public function overwriteOperationName($newOperationName)
     /**
      * {@inheritdoc}
      */
-    public function setTags(array $tags)
+    public function setTag($key, $value)
     {
     }
 

src/OpenTracing/Span.php

@@ -45,17 +45,19 @@ public function finish($finishTime = null, array $logRecords = []);
     public function overwriteOperationName($newOperationName);
 
     /**
-     * Sets tags to the Span in key => value format, key must be a string and tag must be either
-     * a string, a boolean value, or a numeric type.
+     * Adds a tag to the span.
+     *
+     * If there is a pre-existing tag set for key, it is overwritten.
      *
      * As an implementor, consider using "standard tags" listed in {@see \OpenTracing\Tags}
      *
      * If the span is already finished, a warning should be logged.
      *
-     * @param array $tags
+     * @param string $key
+     * @param string|bool|int|float $value
      * @return void
      */
-    public function setTags(array $tags);
+    public function setTag($key, $value);
 
     /**
      * Adds a log record to the span in key => value format, key must be a string and tag must be either