This series adds slow query logging capability to alternator. Queries which last longer than the specified threshold are logged in `system_traces.node_slow_log` and traced.
In order to be better prepared for https://github.com/scylladb/scylla/issues/2572, this series also expands the tracing API to allow custom key-value params and adds a custom `alternator_op` parameter to the slow node log. This information can also be deduced from the tracing session id by consulting the system_traces.events table, but https://github.com/scylladb/scylla/issues/2572 's assumption is that this tracing might not always be available in the future.
This series comes with a simple test case which checks if operation logs indeed end up in `system_traces.node_slow_log`.
Tests:
unit(dev, alternator pytest)
manual: verified that no operations are logged if slow query logging is disabled; verified that operations that take less time than the threshold are not logged; verified with test_batch.py::test_batch_write_item_large that a large-enough operation is indeed logged and traced.
Fixes#8292
Example trace:
```cql
cqlsh> select parameters, duration from system_traces.node_slow_log where start_time=b7a44589-8711-11eb-8053-14c6c5faf955;
parameters | duration
---------------------------------------------------------------------------------------------+----------
{'alternator_op': 'DeleteTable', 'query': '{"TableName": "alternator_Test_1615979572905"}'} | 75732
```
Closes#8298
* github.com:scylladb/scylla:
alternator: add test for slow query logging
alternator: allow enabling slow query logging
tracing: allow providing a custom session record param
42169b2eef