fix(autoware_ekf_localizer): publish diagnostics when ekf_dt is too slow

[RyuYamamoto]

Description

• Output warning diagnostics when ekf period is too slow.

This PR will be merged after the following PR has been merged.
#374

Related links

#374

Parent Issue:

• Link

How was this PR tested?

Running Autoware and confirmed that ekf_period has been added to the diagnostics of ekf_localizer.

Notes for reviewers

None.

Interface changes

None.

Effects on system behavior

None.

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[youtalk]

You might think like a minor effort to reduce dynamic memory usage, but let’s make sure to declare and use constexpr strings (char * or string_view).
This is second time to mention that. #374 (review)

[youtalk]

@RyuYamamoto Could you respond this?

[RyuYamamoto]

@youtalk I apologize for the late reply.

I'd like to confirm one point regarding your comment.
When you mentioned that it might be better to use constexpr, were you referring to the parts in the check_ekf_period function where string values are being assigned, for example?

https://github.com/RyuYamamoto/autoware_core/blob/feat/add_diagnostics/localization/autoware_ekf_localizer/src/diagnostics.cpp#L74
https://github.com/RyuYamamoto/autoware_core/blob/feat/add_diagnostics/localization/autoware_ekf_localizer/src/diagnostics.cpp#L77

[YamatoAndo]

@RyuYamamoto Please update the readme the-conditions-that-result-in-a-warn-state