Add ctime header to metrics_exporter.cc

[ThomsonTan]

Fixes #2186

Header file ctime is needed for using std::time_t in std::strftime.

Changes

Please provide a brief description of the changes here.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[marcalff]

LGTM, assuming minor cleanup for clang-format.

Thanks for the fix.

[codecov]

Codecov Report

Merging #2187 (e040082) into main (d233b9b) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2187   +/-   ##
=======================================
  Coverage   87.52%   87.52%           
=======================================
  Files         169      169           
  Lines        4887     4887           
=======================================
  Hits         4277     4277           
  Misses        610      610           

Impacted Files	Coverage Δ
exporters/ostream/src/metric_exporter.cc	91.81% <ø> (ø)