[fix][fn] Fix Go function incorrectly using nanoseconds for timestamp

[Dream95]

Motivation

Go function runtime recorded last_invocation and exception timestamps using time.UnixNano().

Modifications

• Change setLastInvocation(), addUserException(), and addSysException() in
  pulsar-function-go/pf/stats.go to use UnixMilli() instead of UnixNano().

Verifying this change

• Make sure that the change passes the CI checks.

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

[Dream95]

/pulsarbot rerun-failure-checks