Add wall clock gauge to proxy metrics

[sfleen]

Currently, it's not possible to accurately calculate the time remaining before the proxy certificate expires, since the proxy didn't expose its view of what the time is. This exposes the wall clock time (rounded to the nearest second) as a gauge so downstream consumers can calculate the offsets for cert expiry, as well as any other offsets for timestamps we provide.

Example metrics output:

$ curl http://whoami.default:4191/metrics | grep process_clock_time
...
# HELP process_clock_time_seconds Current system time for this proxy.
# TYPE process_clock_time_seconds gauge
# UNIT process_clock_time_seconds seconds
process_clock_time_seconds 1725565062.0

Fixes #12413

[codecov]

Codecov Report

Attention: Patch coverage is 31.57895% with 13 lines in your changes missing coverage. Please review.

Project coverage is 68.22%. Comparing base (96124bc) to head (bb617f1).
Report is 421 commits behind head on main.

Files with missing lines	Patch %	Lines
linkerd/metrics/src/process.rs	31.57%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3176      +/-   ##
==========================================
+ Coverage   67.68%   68.22%   +0.54%     
==========================================
  Files         332      353      +21     
  Lines       15158    16551    +1393     
==========================================
+ Hits        10259    11292    +1033     
- Misses       4899     5259     +360     

Files with missing lines	Coverage Δ
linkerd/metrics/src/lib.rs	100.00% <ø> (ø)
linkerd/metrics/src/process.rs	71.42% <31.57%> (-9.59%)	⬇️

... and 37 files with indirect coverage changes

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[olix0r]

LGTM 🎉

[cratelyn]

🌮 🌮 🌮