[Internal] Design Docs: Adds Design Document for Client Telemetry (#3590)

* sdk design for client telemetry

* Otel design

* update optel design

* added more nformation

* updated ct design

* remove otel design

* Client Telemetry Design

* update typos

* fix typos

* fix typos

* added limitation

* updated docs

* updated doc

* updated text

* Update docs/observability.md

Co-authored-by: Kiran Kumar Kolli <kirankk@microsoft.com>

* Update docs/observability.md

Co-authored-by: Kiran Kumar Kolli <kirankk@microsoft.com>

* Update docs/observability.md

Co-authored-by: Kiran Kumar Kolli <kirankk@microsoft.com>

* Update docs/observability.md

Co-authored-by: Kiran Kumar Kolli <kirankk@microsoft.com>

* Update docs/observability.md

Co-authored-by: Kiran Kumar Kolli <kirankk@microsoft.com>

* move stuff here and there.

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Co-authored-by: Kiran Kumar Kolli <kirankk@microsoft.com>

docs/observability.md

@@ -34,3 +34,74 @@ flowchart TD
     SendResponse --> OperationCall
 
 ```
+
+## Send telemetry from SDK to service (Private Preview)
+
+### Introduction
+When opted-in CosmosDB SDK collects below aggregated telemetry data every 10 minutes to Azure CosmosDB service. 
+1. Operation(CRUD APIs) Latencies and Request Units (RUs).
+2. Metadata caches (ex: CollectionCache) miss statistics
+3. Client System Usage (during an operation) :
+    * CPU usage
+    * Memory Usage
+    * Thread Starvation
+    * Network Connections Opened (only TCP Connections)
+4. TOP 10 slower network interactions
+
+> Note: We don't collect any PII data as part of this feature.
+
+### Benefits
+Enabling this feature provides numerous benefits. The telemetry data collected will allow us to identify and address potential issues. This results in a superior support experience and ensures that some issues can even be resolved before they impact your application. In short, customers with this feature enabled can expect a smoother and more reliable experience.
+
+### Impact of this feature enabled
+* _Latency_: Customer should not see any impact on latency.
+* _Total RPS_: It depends on the infrastructure the application using SDK is hosted on among other factors but the impact should not exceed 10%.
+* _Any other impact_: Collector needs around 18MB of in-memory storage to hold the data and this storage is always constant (it means it doesn't grow, no matter how much data we have)
+* Benchmark Numbers: https://github.com/Azure/azure-cosmos-dotnet-v3/blob/master/Microsoft.Azure.Cosmos/tests/Microsoft.Azure.Cosmos.Performance.Tests/Contracts/BenchmarkResults.json
+
+### Components
+
+**Telemetry Job:** Background task which collects the data and sends it to a Azure CosmosDB service every 10 minutes.
+
+**Collectors:** In-memory storage which keeps the telemetry data collected during an operation. There are 3 types of collectors including:
+* _Operational Data Collector_: It keeps operation level latencies and request units.
+* _Network Data Collector_: It keeps all the metrics related to network or TCP calls. It has its own Sampler which sample-in only slowest TCP calls for a particular replica.
+* _Cache Data Collector_: It keeps all the cache call latencies. Right now, only collection cache is covered.
+
+**Get VM Information**: 
+
+- Azure VM: [Azure Instance Metadata](https://learn.microsoft.com/azure/virtual-machines/instance-metadata-service?tabs=windows) call. 
+- Non-Azure VM: We don't collect any other information except VMID which will a Guid or Hashed Machine Name.
+
+**Processor**: Its responsibility is to get all the data and divide it into small chunks (<2MB) and send each chunk to the Azure CosmosDB service.
+
+```mermaid
+flowchart TD
+    subgraph TelemetryJob[Telemetry Background Job]
+        subgraph Storage[In Memory Storage or Collectors]
+            subgraph NetworkDataCollector[Network Data Collector]
+                TcpDatapoint(Network Request Datapoint) --> NetworkHistogram[(Histogram)]
+                DataSampler(Sampler)
+            end
+            subgraph DataCollector[Operational Data Collector]
+                OpsDatapoint(Operation Datapoint) --> OperationHistogram[(Histogram)]
+            end
+            subgraph CacheCollector[Cache Data Collector]
+                CacheDatapoint(Cache Request Datapoint) --> CacheHistogram[(Histogram)]
+            end
+        end
+        subgraph TelemetryTask[Telemetry Task Every 10 min]
+            CacheAccountInfo(Cached Account Properties) --> VMInfo
+            VMInfo(Get VM Information) --> CollectSystemUsage
+            CollectSystemUsage(Record System Usage Information) --> GetDataFromCollector
+        end
+        subgraph Processor
+            GetDataFromCollector(Fetch Data from Collectors) --> Serializer
+            Serializer(Serialize and divide the Payload) --> SendCTOverHTTP(Send Data over HTTP to Service)
+        end
+        Storage --> |Get Aggregated data|GetDataFromCollector
+    end
+```
+
+### Limitations
+1. AAD Support is not available.