[clangd] Add a metric for tracking memory usage

[kadircet]

• Rebase

[kadircet]

• Rebase and introduce helper for recording a MemoryTree.

[kadircet]

• Move MemoryTree recording from Trace into MemoryTree as suggested in D88417.

[sammccall]

clang-tools-extra/clangd/support/MemoryTree.h

68 ↗

(On Diff #297172)

can we make this a non-member function and take the metric to write into as a parameter?

Putting this here for reuse seems OK to me but it's not an essential part of MemoryTree itself.
In particular I'd like to avoid baking magic metrics into support/.

(I know we have one already for fallback latency recording, but that seems much costlier to avoid as there are many callsites. Here we expect... 2?)

[kadircet]

• Make traversal a free function and take metric to populate as a parameter.

clang-tools-extra/clangd/support/Trace.h

struct Metric {
   const llvm::StringLiteral Name;
   const MetricType Type;
   /// Indicates what measurement labels represent, e.g. "operation_name" for a
   /// metric tracking latencies. If non empty all measurements must also have a
   /// non-empty label.
   const llvm::StringLiteral LabelName;
 };
 
+/// Convenient helper for collecting memory usage metrics from across multiple
+/// components. Results are recorded under a metric named "memory_usage".
+void recordMemoryUsage(double Value, llvm::StringRef ComponentName);
+
 /// A consumer of trace events and measurements. The events are produced by
 /// Spans and trace::log, the measurements are produced by Metrics::record.
 /// Implementations of this interface must be thread-safe.
 class EventTracer {
 public:
   virtual ~EventTracer() = default;
 
   /// Called when event that has a duration starts. \p Name describes the event.

clang-tools-extra/clangd/support/Trace.cpp

void Metric::record(double Value, llvm::StringRef Label) const {
   assert((LabelName.empty() == Label.empty()) &&
          "recording a measurement with inconsistent labeling");
   T->record(*this, Value, Label);
 }
 
 Context EventTracer::beginSpan(llvm::StringRef Name, llvm::json::Object *Args) {
   return Context::current().clone();
 }
+
+void recordMemoryUsage(double Value, llvm::StringRef ComponentName) {
+  static constexpr Metric MemoryUsage("memory_usage", Metric::Value,
+                                      "component_name");
+
+  MemoryUsage.record(Value, ComponentName);
+}
 } // namespace trace
 } // namespace clangd
 } // namespace clang