Add aws prometheus remote write exporter

[amanbrar1999]

Description:
This PR introduces a new exporter that leverages the existing Prometheus Remote Write Exporter in the core collector repo, and adds AWS Sigv4 authentication logic to it.

The idea here is to add an interceptor to the http client that is used to initialize the Prometheus Remote Write Exporter struct. The interceptor handles the Sigv4 logic, and the rest of the logic is identical to the core exporter.

The way this is designed, any changes in the core exporter's config will need to be duplicated here, however any other changes in the core exporter should be fine.

This PRs size is significantly bloated by the go summary file

Link to tracking Issue:
No direct issue for this exists but it is related to vendor specific code discussion: open-telemetry/oteps#141

Testing:
Unit tests have been added for the factory and config. Testing the authentication logic has been done manually by setting up a pipeline to communicate with AWS services.

Documentation:
A thorough README.md is included, as well as all example configuration in testdata/config.yaml

-- cc @alolita

[kbrockhoff]

LGTM

[bogdandrutu]

@amanbrar1999 any update on this?

[amanbrar1999]

@bogdandrutu We are reviewing the design of this component internally, I will convert to draft for now and reopen when ready

[bogdandrutu]

@amanbrar1999 I am fine if you want to have this discussion here, now that I have context on the change, up to you

[amanbrar1999]

@bogdandrutu I have embedded the factories, but i'm not sure if it has become as simple as you expected. The closure function can not be added in CreateDefaultConfig() because the user specified configuration is not yet present at that stage, hence I needed to also override CreateMetricsExporter(). I also had to override Type() because otherwise it took the type of the embedded exporter.

CreateMetricsExporter() cannot be simplified by calling CreateMetricsExporter() of the embedded factory because we cannot pass in a superset of the embedded factory's config, hence this function remains similar to how it was before

[bogdandrutu]

@bogdandrutu I have embedded the factories, but i'm not sure if it has become as simple as you expected. The closure function can not be added in CreateDefaultConfig() because the user specified configuration is not yet present at that stage, hence I needed to also override CreateMetricsExporter().

Not sure, in the closure you save a pointer to the config, and the closure runs after the config parsing, am I missing something?

I also had to override Type() because otherwise it took the type of the embedded exporter.

True

CreateMetricsExporter() cannot be simplified by calling CreateMetricsExporter() of the embedded factory because we cannot pass in a superset of the embedded factory's config, hence this function remains similar to how it was before

Why? You can pass:

oCfg := cfg.(*Config)
...
return af.ExporterFactory(&oCfg.Config, ....)

[amanbrar1999]

@bogdandrutu It all makes sense and works now, overriding Type() as well as the last point you made was all that was required to add to your code snippet for it to work since we needed to ensure only the embedded config was passed into the embedded CreateMetricsExporter() function. No more code is being copied over from the core exporter into this factory now with these changes.