[cmd/telemetrygen] Export telemetrygen functions for testing

[Erog38]

Description

This PR addresses #36984 in order to open up telemetrygen to be used by golang tests and generate varying amounts of telemetry in code to support use cases which don't rely on external tools.

The following major changes were made to clear the footprint for how this API can be used:

1. Moved metrics, traces, and logs under pkg instead of internal.
2. Unexported the run function for each test suite so the entrypoint is only the Start function
3. Moved config defaults from pflags into a SetDefaults function which is exported alongside NewConfig
   for changed packages in order for users to get the same sane defaults as if run from the command line.
4. Adjusted the pflags to use the existing default values rather than duplicate their own removing default config duplication
5. Added E2E tests for Metrics and Logs and adjusted Traces to use the new API so it stays consistent.
6. Adjusted log and metric exporter instantiation slightly due to a race condition with an err variable which was only caught by the new E2E tests.

Testing

Added to the E2E tests to ensure consistent API specs.

[Erog38]

This goes hand in hand with #37003 in order to help own and maintain this API. I read about chloggen, but haven't used it before so I copied the template and made a version manually. I hope I did that properly.

[codecov]

Codecov Report

Attention: Patch coverage is 15.17857% with 190 lines in your changes missing coverage. Please review.

Project coverage is 79.57%. Comparing base (1629eda) to head (a719b82).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/telemetrygen/pkg/logs/logs.go	8.88%	39 Missing and 2 partials ⚠️
cmd/telemetrygen/pkg/metrics/metrics.go	8.88%	39 Missing and 2 partials ⚠️
cmd/telemetrygen/internal/common/config.go	0.00%	29 Missing ⚠️
cmd/telemetrygen/pkg/traces/config.go	15.15%	28 Missing ⚠️
cmd/telemetrygen/pkg/logs/config.go	35.13%	24 Missing ⚠️
cmd/telemetrygen/pkg/metrics/config.go	0.00%	18 Missing ⚠️
cmd/telemetrygen/pkg/metrics/metrics_types.go	0.00%	5 Missing ⚠️
cmd/telemetrygen/pkg/traces/traces.go	33.33%	2 Missing ⚠️
cmd/telemetrygen/config.go	75.00%	1 Missing ⚠️
cmd/telemetrygen/pkg/metrics/worker.go	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #37044      +/-   ##
==========================================
- Coverage   79.60%   79.57%   -0.03%     
==========================================
  Files        2250     2250              
  Lines      211890   211951      +61     
==========================================
- Hits       168676   168668       -8     
- Misses      37540    37608      +68     
- Partials     5674     5675       +1     

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[Erog38]

Interestingly much of the code stated as not covered in the patch is covered in the e2e tests, I suspect that they don't contribute to the coverage since it's technically a different Go module?

[Erog38]

@mx-psi, @codeboten, sorry to ping both of you, but if you have a moment in your day I'd love to get your thoughts on this implementation.

I did what I could to create a sane first API to maintain and use. Any suggestions are welcome! 😄

I did consider using Options hooks so it would look something like:

Start(
    WithEndpoint("localhost:1234"),
    WithRunDuration(5 * time.Second),
)

But I thought it wouldn't give much more flexibility than just:

mc := metrics.NewConfig()
mc.Endpoint = "localhost:1234"
mc.RunDuration = 5 * time.Second
err := metrics.Start(mc)

Which was simpler to implement and support