fix(ddsql): correct wrong DDSQL time-series help example

[roippi]

Summary

The pup ddsql time-series example in --help queried a bare metrics table, which is not a valid DDSQL dataset. Anyone (or more likely anyone's agent) copying the example verbatim hits an HTTP 400. This swaps it for a verified dd.metrics_timeseries(...) query, the spec-correct way to read metrics via DDSQL.

Changes

• Fixed the time-series example in the ddsql command help (src/main.rs:1083):
  • Before: SELECT avg(system.cpu.user) FROM metrics GROUP BY host
  • After: SELECT timestamp, value, tags->'host' AS host FROM dd.metrics_timeseries('avg:system.cpu.user{*} by {host}')
• Dropped the misleading --interval 300000 flag from the example — metric resolution comes from the dd.metrics_timeseries(...) call, not that flag.

Testing

• Reproduced the original failure against the live API:

$ pup ddsql time-series --query "SELECT avg(system.cpu.user) FROM metrics GROUP BY host" --from 1h
#Error: ... (HTTP 400): {"errors":[{"title":"Bad Request","detail":"depends on non-existent dataset "metrics""}]}

• Verified the replacement returns real per-host timeseries:

$ pup ddsql time-series --query "SELECT timestamp, value, tags->'host' AS host FROM dd.metrics_timeseries('avg:system.cpu.user{*} by {host}')" --from 1h
#[ { "host": "dd-agent7", "timestamp": 1781725600000, "value": 0.1668... }, ... ]

• cargo check passes (the doc comment feeds clap's verbatim_doc_comment derive).

[platinummonkey]

I pulled this commit in and signed it in #605 so we can get the signed commit requirement, thanks for the fix!