Create client on Start instead of Creation

[dashpole]

This unblocks open-telemetry/opentelemetry-collector-contrib#30524 and open-telemetry/opentelemetry-collector-contrib#30527. Components are supposed to perform retry-able steps (which might later succeed), like fetching auth tokens during Start(), rather than during creation. This PR moves getting default credentials and creating clients to the Start() function.

[codecov]

Codecov Report

Attention: Patch coverage is 20.65217% with 73 lines in your changes are missing coverage. Please review.

Project coverage is 59.93%. Comparing base (0fe89f6) to head (29fc64d).

Files	Patch %	Lines
exporter/collector/metrics.go	0.00%	40 Missing and 1 partial ⚠️
exporter/collector/logs.go	0.00%	25 Missing ⚠️
exporter/collector/traces.go	63.15%	5 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #817      +/-   ##
==========================================
- Coverage   60.07%   59.93%   -0.14%     
==========================================
  Files          56       56              
  Lines        5854     5886      +32     
==========================================
+ Hits         3517     3528      +11     
- Misses       2186     2205      +19     
- Partials      151      153       +2     

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