Proposal for cloudprovider storage metrics

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+# Cloud Provider (specifically GCE and AWS) metrics for Storage API calls
+
+## Goal
+
+Kubernetes should provide metrics such as - count & latency percentiles
+for cloud provider API it uses to provision persistent volumes.
+
+In a ideal world - we would want these metrics for all cloud providers
+and for all API calls kubernetes makes but to limit the scope of this feature
+we will implement metrics for:
+
+* GCE
+* AWS
+
+We will also implement metrics only for storage API calls for now. This feature
+does introduces hooks into kubernetes code which can be used to add additonal metrics
+but we only focus on storage API calls here.
+
+## Motivation
+
+* Cluster admins should be able to monitor Cloud API usage of Kubernetes. It will help
+  them detect problems in certain scenarios which can blow up the API quota of Cloud
+  provider.
+* Cluster admins should also be able to monitor health and latency of Cloud API on
+  which kubernetes depends on.
+
+## Implementation
+
+### Metric format and collection
+
+Metrics emitted from cloud provider will fall under category of service metrics
+as defined in [Kubernetes Monitoring Architecture](https://github.com/kubernetes/community/blob/master/contributors/design-proposals/monitoring_architecture.md).
+
+
+The metrics will be emitted using [Prometheus format](https://prometheus.io/docs/instrumenting/exposition_formats/) and available for collection
+from `/metrics` HTTP endpoint of kubelet, controller etc. All Kubernetes core components already emit
+metrics on `/metrics` HTTP endpoint. This proposal merely extends available metrics to include Cloud provider metrics as well.
+
+
+Any collector which can parse Prometheus metric format should be able to collect
+metrics from these endpoints.
+
+A more detailed description of monitoring pipeline can be found in [Monitoring architecture] (https://github.com/kubernetes/community/blob/master/contributors/design-proposals/monitoring_architecture.md#monitoring-pipeline) document.
+
+
+#### Metric Types
+
+Since we are interested in count(or rate) and latency percentile metrics of API calls Kubernetes is making to
+the external Cloud Provider - we will use [Histogram](https://prometheus.io/docs/practices/histograms/) type for
+emitting these metrics.
+
+We will be using `HistogramVec` type so as we can attach dimensions at runtime. Whenever available
+`namespace` will reported as a dimension with the metric.
+
+### GCE Implementation
+
+For GCE we simply use `gensupport.RegisterHook()` to register a function which will be called
+when request is made and response returns.
+
+To begin with we will start emitting following metrics for GCE. Because these metrics are of type
+`Summary` - both count and latency will be automatically calculated.
+
+1. gce_instance_list
+2. gce_disk_insert
+3. gce_disk_delete
+4. gce_attach_disk
+5. gce_detach_disk
+6. gce_list_disk
+
+A POC implementation can be found here - https://github.com/kubernetes/kubernetes/pull/40338/files
+
+### AWS Implementation
+
+For AWS currently we will use wrapper type `awsSdkEC2` to intercept all storage API calls and
+emit metric datapoints.  The reason we are not using approach used for `aws/log_handler` is - because AWS SDK doesn't uses Contexts and hence we can't pass custom information such as API call name or namespace to record with metrics.
+
+To begin with we will start emitting following metrics for AWS:
+
+1. aws_attach_volume
+2. aws_create_tags
+3. aws_create_volume
+4. aws_delete_volume
+5. aws_describe_instance
+6. aws_describe_volume
+7. aws_detach_volume