fix(dns): preserve pod names in histogram rows

[immanuwell]

Keeps pod_name on DNS histogram rows.

The default client deployment has 4 pods. With --db, histogram rows got a NULL pod name, so joins with result rows duplicated data

Repro: run cd dns && python py/run_perf.py --dns-server coredns --params params/coredns/test.yaml --db out/db, then join histograms and results on run and subrun.
Before this fix, every histogram row joins to every client result row

Test: make verify-all-dev and DNS Python unit tests

[kubernetes-prow]

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[kubernetes-prow]

Hi @immanuwell. Thanks for your PR.

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[immanuwell]

@bowei @shyamjvs please take a look