Feat/Add sendgrid_exporter to have available mailing statistics

[slopezz]

Related to https://github.com/3scale/platform/issues/913

This PR adds support for sendgrid_exporter (https://github.com/chatwork/sendgrid-stats-exporter) to prometheus-exporter-operator:

• Added possibility to deploy a sendgrid_exporter using PrometheusExporter type sendgrid (to monitor mailing statistics)

• Added Grafana Dashboard to view mailing stats:

  • On top: monthly/daily aggregated stats which are reset on a montly/daily basic, depending on the value from CR field sendgridAccumulatedMetrics (by default True, which means monthly accumulated metrics)
  • On bottom: Specific rates based on interval (default is hourly rate)

• Added a full example of what is needed to enable the exporter:

  • A single PrometheusExporter type sendgrid object
  • A single Secret containing SendgridAPI credentials

• Adds documentation about the full provided example and available CR fields for exporter customization

• Add examples of possible prometheus rules (alerts) using sendgrid metrics (like requests, spam, blocks...).

[slopezz]

/lgtm

[3scale-robot]

@slopezz: you cannot LGTM your own PR.

In response to [this](https://github.com/3scale-ops/prometheus-exporter-operator/pull/37#issuecomment-1203877144): >/lgtm Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[slopezz]

/approve

[slopezz]

/approve

[3scale-robot]

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