Drop usage of custom Prometheus exporters forks

[marctc]

Currently we are using replace directive in go.mod for a specific subset of Prometheus exporters.
These forks were initially created to make them work with the Agent as some of the exporters need to be configure via command line flag (like windows exporter) or don't provide a way to initialize a collector from an external package.

The main problem is that the forks get outdated at some point, making harder to keep up to date (merge upstream dependencies is not always easier) and frequently unattended.

We should work with upstream exporters to port changes there (whenever is possible). These are the following exporters that need to be addressed:

Tasks

• Get changes of oracledb_exporter merged upstream and remove replace in go.mod agent#3415
• Get changes of dnsmasq_exporter merged upstream and remove replace in go.mod agent#3416
• Get changes of github-exporter merged upstream and remove replace in go.mod agent#3417
• Get changes of process-exporter merged upstream and remove replace in go.mod agent#3418
• Get changes of postgres_exporter merged upstream and remove replace in go.mod agent#3419
• Get changes of windows_exporter merged upstream and remove replace in go.mod agent#3420
• Get changes of mysqld_exporter merged upstream and remove replace in go.mod #465
• Get changes of snmp_exporter merged upstream and remove replace in go.mod agent#3421
• Get changes of azure-metrics-exporter merged upstream and remove replace in go.mod agent#3424
• Use original fork of kafka_exporter #464
• Improve Github Integration agent#1845

Related #548

[rfratto]

Hi there 👋

On April 9, 2024, Grafana Labs announced Grafana Alloy, the spirital successor to Grafana Agent and the final form of Grafana Agent flow mode. As a result, Grafana Agent has been deprecated and will only be receiving bug and security fixes until its end-of-life around November 1, 2025.

To make things easier for maintainers, we're in the process of migrating all issues tagged variant/flow to the Grafana Alloy repository to have a single home for tracking issues. This issue is likely something we'll want to address in both Grafana Alloy and Grafana Agent, so just because it's being moved doesn't mean we won't address the issue in Grafana Agent :)