alternator: fix tracing GetItem
alternator: fix tracing GetItem

The GetItem request did not pass the trace state properly,
which resulted in having almost empty traces.

Refs #6891

Tests: manual:

Before:
 session_id                           | event_id                             | activity                                                                                                               | scylla_parent_id | scylla_span_id  | source    | source_elapsed | thread
--------------------------------------+--------------------------------------+------------------------------------------------------------------------------------------------------------------------+------------------+-----------------+-----------+----------------+---------
 57995da0-cce4-11ea-97ea-000000000000 | 579971c4-cce4-11ea-97ea-000000000000 |                                                                                                                GetItem |                0 | 131309406144163 | 127.0.0.1 |              0 | shard 0

After:
 session_id                           | event_id                             | activity                                                                                                               | scylla_parent_id | scylla_span_id  | source    | source_elapsed | thread
--------------------------------------+--------------------------------------+------------------------------------------------------------------------------------------------------------------------+------------------+-----------------+-----------+----------------+---------
 57995da0-cce4-11ea-97ea-000000000000 | 579971c4-cce4-11ea-97ea-000000000000 |                                                                                                                GetItem |                0 | 131309406144163 | 127.0.0.1 |              0 | shard 0
 57995da0-cce4-11ea-97ea-000000000000 | 57997327-cce4-11ea-97ea-000000000000 | Creating read executor for token -7535857341981351089 with all: {127.0.0.1} targets: {127.0.0.1} repair decision: NONE |                0 | 131309406144163 | 127.0.0.1 |             35 | shard 0
 57995da0-cce4-11ea-97ea-000000000000 | 5799733d-cce4-11ea-97ea-000000000000 |                                                                                            read_data: querying locally |                0 | 131309406144163 | 127.0.0.1 |             38 | shard 0
 57995da0-cce4-11ea-97ea-000000000000 | 57997358-cce4-11ea-97ea-000000000000 |                                                   Start querying the token range that starts with -7535857341981351089 |                0 | 131309406144163 | 127.0.0.1 |             40 | shard 0
 57995da0-cce4-11ea-97ea-000000000000 | 57997579-cce4-11ea-97ea-000000000000 |                                                                                                       Querying is done |                0 | 131309406144163 | 127.0.0.1 |             95 | shard 0
Message-Id: <fc82d7d42f111a801276b210ba242f14a0f9326e@scylladb.com>

avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla