Add metrics

[elliotwms]

Add anonymised metrics to gauge general Pinbot usage. Metrics should not publicly expose any individual user or guild data, instead focusing on things such as:

• Number of installs
• Number of pins

Once we have an internal dashboard then we should look at exposing public snapshots of the data (e.g. via snapshots)

Requirements

• Add metrics to the application
• Gather metrics
• Create an internal dashboard

Acceptance Criteria

This feature will be complete when:

• Metrics which inform us of the usage of Pinbot are gathered and able to be analysed
• Snapshots of these metrics are available

[elliotwms]

Since the webhook rewrite the Lambda now contains invocation metrics which should closely map to calls to the pin command. We could look into adding custom Cloudwatch metrics to add more detail here