[HistogramRegistry] add string handling for template form

[njacazio]

No description provided.

[github-actions]

REQUEST FOR PRODUCTION RELEASES:
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The following labels are available
async-2023-pbpb-apass3
async-2023-pbpb-apass4
async-2023-pp-apass4
async-2024-pp-apass1
async-2022-pp-apass7
async-2024-pp-cpass0

[alibuild]

Error while checking build/O2/fullCI for dc07d4c at 2024-09-25 12:58:

## sw/BUILD/Rivet-latest/log
make[2]: *** [Makefile:544: core.cpp] Error 127
make[1]: *** [Makefile:440: all-recursive] Error 1
make: *** [Makefile:561: all-recursive] Error 1

Full log here.

[jgrosseo]

For me this looks OK. @ktf do you have any comments?

[ktf]

No, I thought I commented already. Sorry about that.