configuration for posting metrics

Configure the following parameters
* metrics_dequeue_delay
* metrics_retry_delay
* metrics_retry_max
* metrics_buffer_size

```
[connection]
metrics_dequeue_delay = 30 # delay for dequeuing from buffer queue
metrics_retry_delay = 60  # delay for retring a request that causes errors
metrics_retry_max = 10    # max numbers of retries for a request that causes errors
metrics_buffer_size = 30  # max numbers of requests stored in buffer queue.
```