[receiver/postgresqlreceiver] fix postgresql.table.size to include indexes and TOAST data

[spathlavath]

Description

• postgresql.table.size was computed using pg_relation_size(), which only measures a table's main data heap
• It silently excludes the table's indexes and TOAST storage (used for large column values)
• Metric is documented as "Disk space used by a table" — the old query didn't match that description
• Fixed by switching the query to pg_total_relation_size() (data + indexes + TOAST)
• Added a regression test since the existing shared integration fixture explicitly excludes this metric from value comparison (IgnoreMetricValues), so nothing previously caught this

Link to tracking issue

Fixes #50914

Testing

• make fmt - clean
• make lint - clean
• make generate - clean, no unexpected diffs
• Full unit test suite - passing
• Full integration test suite - passing, including new test on both postgres:13.18 and postgres:17.2

Documentation

• metadata.yaml description updated to reflect the corrected behavior
• documentation.md regenerated via make generate to match

Authorship

• I, a human, wrote this pull request description myself.

[sv-splunk]

Looks good