feat(instrumentation-pg): propagate context using set application_name

[mhennoch]

Changes

Adds SET application_name as a context propagation mechanism, following the pattern established for SQL Server (SET CONTEXT_INFO) and Oracle (V$SESSION.ACTION).

Why SET application_name?

PostgreSQL instrumentation already supports SQL Commenter, but it has limitations:

• Prepared statements -- SQL Commenter cannot be used with named/prepared queries, as the comment is baked into the statement text and every subsequent execution carries stale trace context.
• Stored procedures -- SQL comments on the outer CALL statement are not visible to queries executed inside the procedure body.

SET application_name operates at the session level and avoids all of these issues.

There is ongoing discussion in the npgsql community about this not being the ideal long-term solution, but no better alternative currently exists for PostgreSQL, and getting to one will take a long time. I will also create a semconv issue about it. This feature is disabled by default, as in the other instrumentations.

Existing and related implementations

• Node.js (tedious/SQL Server): opentelemetry-js-contrib#3141
• Node.js (oracledb): opentelemetry-js-contrib#3226
• Java (PostgreSQL): splunk-otel-java#2540
• Collector: opentelemetry-collector-contrib#44868

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[maryliag]

I haven't checked the code yet, but can you add more tests? I can see that the codecov is failing, so this will need to increase the coverage