fix: pass node label value to OverlayBD_Alive metric render

[SriHarsha001]

What this PR does / why we need it:
The OverlayBD_Alive metric produces malformed Prometheus exposition format output, causing Prometheus-compatible scrapers (e.g. vmagent) to fail parsing the /metrics endpoint. In src/exporter_handler.h, alive.render(1) is called without providing a string argument for the {node} label placeholder. The template renderer places the metric value and timestamp where the node label value should be, producing:
OverlayBD_Alive{node="1.000000 1778265650342
instead of:
OverlayBD_Alive{node=""} 1.000000 1778265650342

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes # This fix stores the machine hostname in ExposeRender and passes it to alive.render(1, node_name.c_str()), producing valid Prometheus output.

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test? No existing tests for the metrics exporter; this is a one-line bug fix to an existing render call.
• Does this change require a documentation update? No.
• Does this introduce breaking changes that would require an announcement or bumping the major version? No. The metric was previously unparseable; this makes it valid.
• Do all new files have an appropriate license header? No new files; only an existing file was modified.