Add information about shard/thread to the tracing

[sylwiaszunejko]

Previously there was no information about shards/threads in tracing. It could be problematic, if we want to test shard/thread related features.

This PR introduces adding information about shard/thread to the tracing. This information could be needed for testing shard awareness. Also it is more in line with cqlsh tracing.

In Cassandra single tracing event looks like that:

session_id                           | event_id                             | activity                                    | source    | source_elapsed | source_port | thread
-------------------------------------+--------------------------------------+---------------------------------------------+-----------+----------------+-------------+-----------------------------
568f2370-26ed-11ee-8120-57607de2775b | 5690aa10-26ed-11ee-8120-57607de2775b |        Parsing SELECT * FROM system.local ; | 127.0.0.1 |           7107 |        null | Native-Transport-Requests-1

where thead could be for example: Native-Transport-Requests-1, ReadStage-2.

Accordingly, in Scylla single tracing event looks like that:

session_id                           | event_id                             | activity                                  | scylla_parent_id | scylla_span_id  | source    | source_elapsed | thread
-------------------------------------+--------------------------------------+-------------------------------------------+------------------+-----------------+-----------+----------------+----------
7b9f82c0-2604-11ee-ad99-17a0af160bf8 | 7b9f9b56-2604-11ee-ad99-17a0af160bf8 |                       Parsing a statement |                0 | 147842476120455 | 127.0.0.2 |              1 |  shard 4

with thread in the form: shard x, where x is a number.

[martin-sucha]

Looks good to me. Thank you!