Add auth parameter on PrometheusRemoteWriteMetricsExporter to enable Custom HTTP authentication like AWS Sigv4

[angelotessaro]

Description

#2177

Fixes # 2177

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

Used custom AWS Sigv4 authentication object to export metrics to Amazon Managed Prometheus service directly, as the following code:

    from opentelemetry.sdk.metrics import MeterProvider
    from opentelemetry.sdk.metrics.export import PeriodicExportingMetricReader
    from opentelemetry.exporter.prometheus_remote_write import (
    PrometheusRemoteWriteMetricsExporter,
)
    from requests_auth_aws_sigv4 import AWSSigV4

    aws_auth = AWSSigV4('aps',
        aws_access_key_id=AWS_ACCESS_KEY_ID,
        aws_secret_access_key=AWS_SECRET_ACCESS_KEY,
        region='eu-west-1',
    )
    exporter = PrometheusRemoteWriteMetricsExporter(
        endpoint='REMOTE_WRITE_ENDPOINT',
        headers={},
        auth=aws_auth, 
    )

    reader = PeriodicExportingMetricReader(exporter, 1000)
    provider = MeterProvider(metric_readers=[reader])

    FlaskInstrumentor().instrument_app(app, meter_provider=provider)

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[linux-foundation-easycla]

• ✅ login: ocelotl / name: Diego Hurtado (7119a86)
• ❌ The email address for the commit (0691243) is not linked to the GitHub account, preventing the EasyCLA check. Consult this Help Article and GitHub Help to resolve. (To view the commit's email address, add .patch at the end of this PR page's URL.) For further assistance with EasyCLA, please submit a support request ticket.

[xrmx]

The callable should be compatible with requests though, so it should take a requests' Request as parameter and set whatever header is required. The problem with that though is that it'll leak the implementation. So maybe is it better to expand the documentation of the exporter to add an example on how to add authentication using just headers instead?

[ocelotl]

@angelotessaro please sign the CLA