Add metrics to the `Storage` implementation

[everettraven]

As a follow up to #113 we need to add metrics to assess the health of the HTTP server. The method we are planning to use for assessing the health of the service is the Apdex Score, which will require the implementation of 2 specific metrics:

• Response time/duration
• Response codes

Acceptance Criteria:

• The Storage implementation is updated to produce the above metrics
• Nice to have: Grafana dashboard to show graphs for the above metrics and the apdex score calculation

[ncdc]

Rather than recording 5xx error counts, it might be better to record all response codes?

[everettraven]

Rather than recording 5xx error counts, it might be better to record all response codes?

Good point. Will update the description

[everettraven]

/assign