Make some document properties extensible (#1163)

The `Documents` domain class currently assumes static corpora and thus
populates all properties eagerly via instance fields. There are cases
however when we want to resolve these attributes lazily because the
respective set of documents is generated at a later stage in the
benchmark and we cannot derive these properties upfront. With this
commit we make the relevant properties extensible so they can be
resolved lazily in subclass implementations.

esrally/track/track.py

@@ -213,9 +213,9 @@ def __init__(self, source_format, document_file=None, document_archive=None, bas
         self.document_archive = document_archive
         self.base_url = base_url
         self.includes_action_and_meta_data = includes_action_and_meta_data
-        self.number_of_documents = number_of_documents
-        self.compressed_size_in_bytes = compressed_size_in_bytes
-        self.uncompressed_size_in_bytes = uncompressed_size_in_bytes
+        self._number_of_documents = number_of_documents
+        self._compressed_size_in_bytes = compressed_size_in_bytes
+        self._uncompressed_size_in_bytes = uncompressed_size_in_bytes
         self.target_index = target_index
         self.target_data_stream = target_data_stream
         self.target_type = target_type
@@ -227,6 +227,30 @@ def has_compressed_corpus(self):
     def has_uncompressed_corpus(self):
         return self.document_file is not None
 
+    @property
+    def number_of_documents(self):
+        return self._number_of_documents
+
+    @number_of_documents.setter
+    def number_of_documents(self, value):
+        self._number_of_documents = value
+
+    @property
+    def uncompressed_size_in_bytes(self):
+        return self._uncompressed_size_in_bytes
+
+    @uncompressed_size_in_bytes.setter
+    def uncompressed_size_in_bytes(self, value):
+        self._uncompressed_size_in_bytes = value
+
+    @property
+    def compressed_size_in_bytes(self):
+        return self._compressed_size_in_bytes
+
+    @compressed_size_in_bytes.setter
+    def compressed_size_in_bytes(self, value):
+        self._compressed_size_in_bytes = value
+
     @property
     def number_of_lines(self):
         if self.includes_action_and_meta_data: