Merge tag 'v1.16.0rc2' into develop

Synapse 1.16.0rc2 (2020-07-02)
==============================

Synapse 1.16.0rc2 includes the security fixes released with Synapse 1.15.2.
Please see [below](https://github.com/matrix-org/synapse/blob/master/CHANGES.md#synapse-1152-2020-07-02) for more details.

Improved Documentation
----------------------

- Update postgres image in example `docker-compose.yaml` to tag `12-alpine`. ([\#7696](https://github.com/matrix-org/synapse/issues/7696))

Internal Changes
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- Add some metrics for inbound and outbound federation latencies: `synapse_federation_server_pdu_process_time` and `synapse_event_processing_lag_by_event`. ([\#7771](https://github.com/matrix-org/synapse/issues/7771))