Remove non-default feature from doc test

[bantonsson]

Changes

Running cargo test --all locally fails in the doc tests with:

---- opentelemetry-semantic-conventions/src/resource.rs - resource (line 16) stdout ----
error[E0425]: cannot find value `SERVICE_NAMESPACE` in module `semconv::resource`
    --> opentelemetry-semantic-conventions/src/resource.rs:25:42
     |
12   |         KeyValue::new(semconv::resource::SERVICE_NAMESPACE, "my-namespace"),
     |                                          ^^^^^^^^^^^^^^^^^ help: a constant with a similar name exists: `SERVICE_NAME`
     |

This removes the SERVICE_NAMESPACE resource from the doc test, since it is now behind the non default feature semconv_experimental, making the doc test fail. It also updates the metrics template to use the build() method instead of the renamed init()

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

All modified and coverable lines are covered by tests ✅

Project coverage is 79.5%. Comparing base (1541a33) to head (6120df3).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2346   +/-   ##
=====================================
  Coverage   79.5%   79.5%           
=====================================
  Files        123     123           
  Lines      21365   21365           
=====================================
  Hits       17004   17004           
  Misses      4361    4361           

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[cijothomas]

Thanks. Need one more eyes to help review before merge.