Fix callback scheduling issues on local time change

[afurche]

Callback scheduling was based on datetime.now(), which internally is based on time.time(). As it uses real system clock, it is susceptible to time changes (e.g. end of daylight saving time), which introduces scheduling issues on such occurrences.

This PR changes the scheduling mechanism, to use time.monotonic() instead, which is based on a monotonic system clock, and is not affected by time changes, which ensures scheduling stays stable through time changes.

[Copilot]

Pull Request Overview

This PR fixes callback scheduling issues that occur during system time changes (such as daylight saving time transitions) by switching from time.time() to time.monotonic() for internal scheduling. The monotonic clock is unaffected by system time adjustments, ensuring stable and reliable callback execution timing.

Key Changes:

• Replaced datetime.now() and timedelta with time.monotonic() and numeric seconds for scheduling
• Updated internal callback intervals to use raw seconds instead of timedelta objects
• Modified type hints to reflect the change from datetime to float timestamps

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
dynatrace_extension/sdk/extension.py	Converted interval constants from timedelta to seconds, updated scheduler to use time.monotonic(), changed internal callback timestamps from datetime to float
dynatrace_extension/sdk/callback.py	Added start_timestamp_monotonic using time.monotonic() and updated get_next_execution_timestamp() to use monotonic time
tests/sdk/test_extension.py	Updated test to use time.monotonic() instead of datetime.now() for _next_heartbeat
dynatrace_extension/about.py	Bumped version from 1.7.2 to 1.7.3

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