fix: Align OpenTelemetry attribute coverage with database-spans semconv

[chmodas]

The db.client.operation.duration histogram previously carried only connection-level attributes (db.system.name, db.namespace). Annotation-derived attributes (db.operation.name, db.collection.name, db.query.summary, db.stored_procedure.name) and error-path attributes (error.type, plus db.response.status_code for sqlx::Error::Database) are now appended to metric_attrs in lock-step with the span attribute set, so dashboards can slice DB latency by operation verb, target collection, or error class – the dimensions OTel's database-spans semconv recommends. InstrumentedStream gains a single-error latch so streams that yield multiple Err items before terminating cannot append duplicate error.type / db.response.status_code keys.

Three additional attributes from the OTel database-spans recommended set now surface on every span and per-operation metric: db.client.connection.pool.name (previously only on the db.client.connection.count gauge; now on every span and the rest of the db.client.connection.* family via the shared connection-attribute set), network.protocol.name (per-backend default via the new Database::DEFAULT_NETWORK_PROTOCOL_NAME constant; overridable via PoolBuilder::with_network_protocol_name), and network.transport (set explicitly via PoolBuilder::with_network_transport; the wrapper does not infer transport from the connect string).

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.95%. Comparing base (edc9e30) to head (df763be).
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Additional details and impacted files

@@            Coverage Diff             @@
##            trunk      #32      +/-   ##
==========================================
+ Coverage   99.90%   99.95%   +0.05%     
==========================================
  Files          12       12              
  Lines        2034     2164     +130     
==========================================
+ Hits         2032     2163     +131     
+ Misses          2        1       -1     

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