[Python Otel] Fix Segfault caused by CallTracer method lifetime issue (#37329)

We're seeing a segfault issue in observability tests:
```
2024-07-26T09:09:18.422255153Z *** SIGSEGV received at time=1721984958 on cpu 0 ***
2024-07-26T09:09:18.424985750Z PC: @     0x7e1acccb71c9  (unknown)  (unknown)
2024-07-26T09:09:18.425333774Z     @     0x7e1ac714ed8c         64  absl::lts_20240116::WriteFailureInfo()
2024-07-26T09:09:18.425356717Z     @     0x7e1ac714ea15        272  absl::lts_20240116::AbslFailureSignalHandler()
2024-07-26T09:09:18.425368880Z     @     0x7e1accb98050       1584  (unknown)
2024-07-26T09:09:18.426117382Z     @     0x7e1ac77f458c        112  absl::lts_20240116::string_view::operator std::__cxx11::basic_string<><>()
2024-07-26T09:09:18.426647368Z     @     0x7e1ac78008df        688  grpc_observability::PythonOpenCensusCallTracer::PythonOpenCensusCallAttemptTracer::RecordEnd()
```

It points to `absl::string_view::operator std::__cxx11::basic_string<>()` which indicates the issue might be related to string conversion.

The most probable cause is that the `parent_->method_` string object is being destroyed before the `std::string` conversion is completed or used by `emplace_back`:

b056bc41d3/src/python/grpcio_observability/grpc_observability/client_call_tracer.cc (L325-L326)

Since it's difficult to manage the lifecycle of `method` in Python/Cython, this PR changes `method_` and `traget_` from `absl::string_view` to `std::string` so that they'll always be available.

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Closes #37329

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37329 from XuanWang-Amos:fix_otel_segfault c579022529
PiperOrigin-RevId: 657366034

src/python/grpcio_observability/grpc_observability/client_call_tracer.cc

@@ -88,7 +88,7 @@ void PythonOpenCensusCallTracer::RecordAnnotation(
 
 PythonOpenCensusCallTracer::~PythonOpenCensusCallTracer() {
   if (PythonCensusStatsEnabled()) {
-    context_.Labels().emplace_back(kClientMethod, std::string(method_));
+    context_.Labels().emplace_back(kClientMethod, method_);
     RecordIntMetric(kRpcClientRetriesPerCallMeasureName, retries_ - 1,
                     context_.Labels(), identifier_, registered_method_,
                     /*include_exchange_labels=*/true);  // exclude first attempt
@@ -159,8 +159,8 @@ PythonOpenCensusCallTracer::PythonOpenCensusCallAttemptTracer::
   if (!PythonCensusStatsEnabled()) {
     return;
   }
-  context_.Labels().emplace_back(kClientMethod, std::string(parent_->method_));
-  context_.Labels().emplace_back(kClientTarget, std::string(parent_->target_));
+  context_.Labels().emplace_back(kClientMethod, parent_->method_);
+  context_.Labels().emplace_back(kClientTarget, parent_->target_);
   RecordIntMetric(kRpcClientStartedRpcsMeasureName, 1, context_.Labels(),
                   parent_->identifier_, parent_->registered_method_,
                   /*include_exchange_labels=*/false);
@@ -264,8 +264,8 @@ void PythonOpenCensusCallTracer::PythonOpenCensusCallAttemptTracer::
   }
 
   std::string final_status = absl::StatusCodeToString(status_code_);
-  context_.Labels().emplace_back(kClientMethod, std::string(parent_->method_));
-  context_.Labels().emplace_back(kClientTarget, std::string(parent_->target_));
+  context_.Labels().emplace_back(kClientMethod, parent_->method_);
+  context_.Labels().emplace_back(kClientTarget, parent_->target_);
   context_.Labels().emplace_back(kClientStatus, final_status);
   if (parent_->add_csm_optional_labels_) {
     parent_->labels_injector_.AddXdsOptionalLabels(
@@ -322,8 +322,7 @@ void PythonOpenCensusCallTracer::PythonOpenCensusCallAttemptTracer::
 void PythonOpenCensusCallTracer::PythonOpenCensusCallAttemptTracer::RecordEnd(
     const gpr_timespec& /*latency*/) {
   if (PythonCensusStatsEnabled()) {
-    context_.Labels().emplace_back(kClientMethod,
-                                   std::string(parent_->method_));
+    context_.Labels().emplace_back(kClientMethod, parent_->method_);
     context_.Labels().emplace_back(kClientStatus,
                                    StatusCodeToString(status_code_));
     RecordIntMetric(kRpcClientSentMessagesPerRpcMeasureName,

src/python/grpcio_observability/grpc_observability/client_call_tracer.h

@@ -140,9 +140,9 @@ class PythonOpenCensusCallTracer : public grpc_core::ClientCallTracer {
   PythonCensusContext CreateCensusContextForCallAttempt();
 
   // Client method.
-  absl::string_view method_;
+  std::string method_;
   // Client target.
-  absl::string_view target_;
+  std::string target_;
   PythonCensusContext context_;
   bool tracing_enabled_;
   bool add_csm_optional_labels_;

src/python/grpcio_observability/grpc_observability/python_observability_context.h

@@ -272,17 +272,15 @@ void GenerateClientContext(absl::string_view method, absl::string_view trace_id,
 void GenerateServerContext(absl::string_view header, absl::string_view method,
                            PythonCensusContext* context);
 
-inline absl::string_view GetMethod(const char* method) {
+inline std::string GetMethod(const char* method) {
   if (std::string(method).empty()) {
     return "";
   }
   // Check for leading '/' and trim it if present.
-  return absl::StripPrefix(absl::string_view(method), "/");
+  return std::string(absl::StripPrefix(method, "/"));
 }
 
-inline absl::string_view GetTarget(const char* target) {
-  return absl::string_view(target);
-}
+inline std::string GetTarget(const char* target) { return std::string(target); }
 
 // Fills a pre-allocated buffer with the value for the grpc-trace-bin header.
 // The buffer must be at least kGrpcTraceBinHeaderLen bytes long.