[Metrics API] Rename init to build

[utpilla]

Towards #1699

Changes

• Rename init method to build

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 97.43590% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.1%. Comparing base (8bd529a) to head (5cbf860).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/mod.rs	96.9%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2232   +/-   ##
=====================================
  Coverage   79.1%   79.1%           
=====================================
  Files        121     121           
  Lines      21171   21171           
=====================================
+ Hits       16760   16762    +2     
+ Misses      4411    4409    -2     

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[cijothomas]

the changelog about try_init still refers to "init". lets change that also.

Alternatively, we can make a single changelog entry consolidating the user-facing changes. Something like
Removed try_init method from instrument builders. To build instruments, user the build() method.
This method always return an instrument - in case of no sdk being enabled or instrument validation failed (invalid name etc.), or the instrument is disabled using views, the instrument internally uses a no-op.
eg:
before:
let counter = meter.u64_counter("my_counter").init();
let counter = meter.u64_counter("my_counter").try_init();
after:
let counter = meter.u64_counter("my_counter").build();

[cijothomas]

Merging. The changelog suggestions can be handled in a consolidated way before next release.