Enable 'heartbeat_on_demand_duration' in local/examples

[shlomi-noach]

Description

Followup to #14978, #14980, and #15099. This PR re-enables heartbeat_on_demand_duration in vttablet-up.sh script in examples/local.

This comes at no cost when the throttler is disabled, and makes it possible for the throttler to work based on replication lag.

Future work to change the defaults/default behavior of heartbeat mechanism. See discussion in #15099.

Without this PR, the throttler won't work properly in examples. I therefore want to backport this to all supported versions, just as #14980 was backported.

Related Issue(s)

• Bug Report: Vitess local examples high replica heartbeat lag #14978
• examples: rm heartbeat flags #14980
• Auto-set 5s on-demand heartbeat if --enable_heartbeat is disabled #15099

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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