fix(instrumentation-kafkajs): add missing delcare keyword for metric instruments

[pichlermarc]

Which problem is this PR solving?

See #2804, the declare keyword is missing, which is required for our version of TypeScript (yes, this is a footgun in @opentelemetry/instrumentation). I'm not exactly sure why the tests were not failing here.

Fixes #2804

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.50%. Comparing base (f07dcce) to head (f3fad29).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2805      +/-   ##
==========================================
- Coverage   89.55%   89.50%   -0.05%     
==========================================
  Files         180      180              
  Lines        8719     8719              
  Branches     1767     1767              
==========================================
- Hits         7808     7804       -4     
- Misses        911      915       +4     

Files with missing lines	Coverage Δ
...ode/instrumentation-kafkajs/src/instrumentation.ts	97.57% <ø> (ø)

... and 1 file with indirect coverage changes

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