Fix too many requests error. Add JSON logs.

[sfairat15]

Fixing issues #10 and #9.

Main change

According Microsoft documentation application may create only 30 API calls per minute.
After that threshold application will get Too Many Requests response from API.

Error code 429 - Call count has exceeded rate limits

To enable a consistent experience for all Cost Management subscribers, Cost Management APIs are rate limited. When you reach the limit, you receive the HTTP status code 429: Too many requests. The current throughput limits for our APIs are as follows:

30 calls per minute - It's done per scope, per user, or application.

200 calls per minute - It's done per tenant, per user, or application.

To avoid such errors, this exporter has background job to get data from API.
Received cost data placed in memory cache. Prometheus scriber calls on /metrics get data from cache and get quick response.

In case of Too Many Requests errors, background job waits 1 minute before next calls.

Also

• Add logging in JSON format (use Serilog)
• Use Dodo.HttpClientResiliencePolicies policy library for API calls