Merge pull request #5769 from matrix-org/uhoreg/e2e_cross-signing2-part1

Cross-signing [2/4] - upload/download keys

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+Room and User Statistics
+========================
+
+Synapse maintains room and user statistics (as well as a cache of room state),
+in various tables. These can be used for administrative purposes but are also
+used when generating the public room directory.
+
+
+# Synapse Developer Documentation
+
+## High-Level Concepts
+
+### Definitions
+
+* **subject**: Something we are tracking stats about – currently a room or user.
+* **current row**: An entry for a subject in the appropriate current statistics
+    table. Each subject can have only one.
+* **historical row**: An entry for a subject in the appropriate historical
+    statistics table. Each subject can have any number of these.
+
+### Overview
+
+Stats are maintained as time series. There are two kinds of column:
+
+* absolute columns – where the value is correct for the time given by `end_ts`
+    in the stats row. (Imagine a line graph for these values)
+    * They can also be thought of as 'gauges' in Prometheus, if you are familiar.
+* per-slice columns – where the value corresponds to how many of the occurrences
+    occurred within the time slice given by `(end_ts − bucket_size)…end_ts`
+    or `start_ts…end_ts`. (Imagine a histogram for these values)
+
+Stats are maintained in two tables (for each type): current and historical.
+
+Current stats correspond to the present values. Each subject can only have one
+entry.
+
+Historical stats correspond to values in the past. Subjects may have multiple
+entries.
+
+## Concepts around the management of stats
+
+### Current rows
+
+Current rows contain the most up-to-date statistics for a room.
+They only contain absolute columns
+
+### Historical rows
+
+Historical rows can always be considered to be valid for the time slice and
+end time specified.
+
+* historical rows will not exist for every time slice – they will be omitted
+    if there were no changes. In this case, the following assumptions can be
+    made to interpolate/recreate missing rows:
+    - absolute fields have the same values as in the preceding row
+    - per-slice fields are zero (`0`)
+* historical rows will not be retained forever – rows older than a configurable
+    time will be purged.
+
+#### Purge
+
+The purging of historical rows is not yet implemented.