feat: add barrier align metrics for merge executor

[lmatz]

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What's changed and what's your intention?

dev=> create table t(v1 int);
CREATE_TABLE
dev=> create materialized view m as select * from t;
CREATE_MATERIALIZED_VIEW

Merge is different in the sense that it merges input from all the upstream actors belonging to the same fragment.
Join and DynamicFilter merges input from different upstream fragments

Checklist

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