Prepared statement execute call tracing.

[damour]

Sometimes Doctrine\DBAL\Statement::execute() call can take a long time (because of locks for example).

This PR adds the ability to create a separate span on execute() call.

[allflame]

I understand the premise, but suggested PR changes behavior of the original \Doctrine\DBAL\Connection::prepare method, e.g.
After these changes JaegerConnectionWrapper won't call parent::prepare anymore, thus violating L principle.
Can we achieve similar thing with just substituion of parent::prepare with new JaegerStatementWrapper(parent::prepare, $this, $this->tracer)?

[andrew-demb]

The same problem was resolved in Sentry SDK recently. getsentry/sentry-symfony#548