Add new metrics

[yngvar-antonsson]

• tnt_replication_upstream_id_lag
• tnt_replication_downstream_id_lag
• tnt_vinyl_regulator_blocked_writers
• tnt_net_requests_in_progress_total
• tnt_net_requests_in_progress_current
• tnt_net_requests_in_stream_total
• tnt_net_requests_in_stream_current

I didn't forget about

• [x] Tests
• [x] Changelog
• [x] Documentation (README and rst)

Close #337

[DifferentialOrange]

Damn, I forgot to submit my last review. If anything still seems relevant, please fix it in separate PR