pg: record datastore statement metrics

The three PG tests were also consolidated into one common test script
with different initializations.
pg: Fix for using query events instead of callbacks

When no callback was detected, we were inserting our own. The insertion
itself caused a crash, but on top of that adding a callback modifies the
behavior of the pg module. Instead, we now listen for error or end
events to finish segments.

We also now catch new segments started in event handlers properly.
pg: Remove feature flagging
api: make custom instrumentation return the handle's return value
package.json: Bundle proxying-agent

This was accidentally left unbundled when we incorporated the newer
upstream version in release 1.11.0.
test: Add test to ensure all dependencies are bundled
docs: updated for new version
release: published 1.11.2