Add a metrics of GitHub Apps installation

[whywaita]

This pull request includes significant changes to the GitHub integration and Prometheus metrics collection. The most important changes are the addition of new functions for listing GitHub installations and enhancements to the Prometheus metrics for datastore and GitHub installations.

GitHub Integration Enhancements:

• Added a new function listInstallations in pkg/gh/installation.go to list GitHub installations with caching to improve performance.
• Removed the redundant listInstallations function from pkg/gh/jwt.go as it was moved to pkg/gh/installation.go.

Prometheus Metrics Enhancements:

• Added new Prometheus metrics descriptors for datastore targets and token expiration times in pkg/metric/scrape_datastore.go.
• Updated the scrapeTargets function in pkg/metric/scrape_datastore.go to include the new metrics for datastore targets and token expiration times.
• Introduced a new Prometheus metric descriptor for GitHub installations in pkg/metric/scrape_github.go.
• Added a new function scrapeInstallation in pkg/metric/scrape_github.go to scrape and collect metrics for GitHub installations.
• Updated the Scrape function in pkg/metric/scrape_github.go to include the new scrapeInstallation function.This pull request introduces new functionality for caching GitHub installations and adds new Prometheus metrics for monitoring. The main changes include the addition of a new method to list and cache GitHub installations, the removal of the old method, and the introduction of new Prometheus metrics for both datastore targets and GitHub installations.

Caching GitHub Installations:

• pkg/gh/installation.go: Added a new method listInstallations that caches the result of GitHub installations for one hour to improve performance.
• pkg/gh/jwt.go: Removed the old listInstallations method as it has been replaced by the new method in installation.go.

Prometheus Metrics:

• pkg/metric/scrape_datastore.go: Added a new Prometheus metric datastoreTargetDesc to describe datastore targets and updated the scrapeTargets function to use this new metric. [1] [2]
• pkg/metric/scrape_github.go: Added a new Prometheus metric githubInstallationDesc to describe GitHub installations and updated the ScraperGitHub to scrape installation metrics using the new scrapeInstallation function. [1] [2] [3]

[gamoutatsumi]

LGTM