fix: Fix a few minor issues with join metrics

[linhr]

Which issue does this PR close?

N/A

Rationale for this change

There are a few minor issues for join metrics:

1. The ExecutionPlan::metrics() method is missing for PiecewiseMergeJoinExec.
2. The metric names for left and right inputs are the same in StreamJoinMetrics, so the user cannot distinguish the metrics in the metric set.

What changes are included in this PR?

This PR fixes the issues above.

Are these changes tested?

N/A

Are there any user-facing changes?

The changes to metric names in StreamJoinMetrics (used by SymmetricHashJoinExec) is potentially user-facing.