Add support for Statistics like total document count or number of documents per mime type

[andygrunwald]

This PR adds the standard paperless statistics (visible on the top right box in the paperless UI) into the export format.

Dependency

This PR depends on hansmi/paperhooks#43

Output

# HELP paperless_statistics_character_count Number of characters stored across the total number of documents.
# TYPE paperless_statistics_character_count gauge
paperless_statistics_character_count 1.1048372e+07
# HELP paperless_statistics_correspondent_count Total number of correspondents.
# TYPE paperless_statistics_correspondent_count gauge
paperless_statistics_correspondent_count 201
# HELP paperless_statistics_document_type_count Total number of document types.
# TYPE paperless_statistics_document_type_count gauge
paperless_statistics_document_type_count 42
# HELP paperless_statistics_documents_file_type_counts Total number of documents per MIME type.
# TYPE paperless_statistics_documents_file_type_counts gauge
paperless_statistics_documents_file_type_counts{mime_type="application/msword"} 2
paperless_statistics_documents_file_type_counts{mime_type="application/pdf"} 1397
paperless_statistics_documents_file_type_counts{mime_type="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"} 6
paperless_statistics_documents_file_type_counts{mime_type="application/vnd.openxmlformats-officedocument.wordprocessingml.document"} 37
paperless_statistics_documents_file_type_counts{mime_type="image/jpeg"} 4
paperless_statistics_documents_file_type_counts{mime_type="text/plain"} 1
# HELP paperless_statistics_documents_inbox_count Total number of documents that have the defined 'Inbox' tag.
# TYPE paperless_statistics_documents_inbox_count gauge
paperless_statistics_documents_inbox_count 273
# HELP paperless_statistics_documents_total Total number of documents.
# TYPE paperless_statistics_documents_total gauge
paperless_statistics_documents_total 1447
# HELP paperless_statistics_storage_path_count Total number of storage pathes.
# TYPE paperless_statistics_storage_path_count gauge
paperless_statistics_storage_path_count 0
# HELP paperless_statistics_tag_count Total number of tags.
# TYPE paperless_statistics_tag_count gauge
paperless_statistics_tag_count 55