[llvm] [llvm]Add a simple Telemetry framework (PR #102323)

[James Henderson via llvm-commits]

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+===========================
+Telemetry framework in LLVM
+===========================
+
+.. contents::
+   :local:
+
+.. toctree::
+   :hidden:
+
+===========================
+Telemetry framework in LLVM
+===========================
+
+Objective
+=========
+
+Provides a common framework in LLVM for collecting various usage and performance
+metrics.
+It is located at `llvm/Telemetry/Telemetry.h`
+
+Characteristics
+---------------
+* Configurable and extensible by:
+
+  * Tools: any tool that wants to use Telemetry can extend and customize it.
+  * Vendors: Toolchain vendors can also provide custom implementation of the
+    library, which could either override or extend the given tool's upstream
+    implementation, to best fit their organization's usage and privacy models.
+  * End users of such tool can also configure Telemetry (as allowed by their
+    vendor).
+
+
+Important notes
+----------------
+
+* There is no concrete implementation of a Telemetry library in upstream LLVM.
+  We only provide the abstract API here. Any tool that wants telemetry will
+  implement one.
+  
+  The rationale for this is that, all the tools in LLVM are very different in
+  what they care about (what/where/when to instrument data). Hence, it might not
+  be practical to have a single implementation.
+  However, in the future, if we see enough common pattern, we can extract them
+  into a shared place. This is TBD - contributions are welcomed.
+
+* No implementation of Telemetry in upstream LLVM shall store any of the
+  collected data due to privacy and security reasons:
+  
+  * Different organizations have different privacy models:
+  
+    * Which data is sensitive, which is not?
+    * Whether it is acceptable for instrumented data to be stored anywhere?
+      (to a local file, what not?)
+      
+  * Data ownership and data collection consents are hard to accommodate from
+    LLVM developers' point of view:
+  
+    * Eg., data collected by Telemetry is not neccessarily owned by the user
+      of an LLVM tool with Telemetry enabled, hence the user's consent to data
+      collection is not meaningful. On the other hand, LLVM developers have no
+      reasonable ways to request consent from the "real" owners.
+
+
+High-level design
+=================
+
+Key components
+--------------
+
+The framework consists of four important classes:
+
+* `llvm::telemetry::Telemeter`: The class responsible for collecting and
+  transmitting telemetry data. This is the main point of interaction between the
+  framework and any tool that wants to enable telemery.
+* `llvm::telemetry::TelemetryInfo`: Data courier
+* `llvm::telemetry::Destination`: Data sink to which the Telemetry framework
+  sends data.
+  Its implementation is transparent to the framework.
+  It is up to the vendor to decide which pieces of data to forward and where
+  to forward them to their final storage.
----------------
jh7370 wrote:

```suggestion
* `llvm::telemetry::Destination`: Data sink to which the Telemetry framework
  sends data.
  Its implementation is transparent to the framework.
  It is up to the vendor to decide which pieces of data to forward and where
  to forward them to for their final storage.
```

https://github.com/llvm/llvm-project/pull/102323