[yugabyte#26521] YSQL: fixed vmodule flag handling

Summary:
After commit f85bbca, vmodule flag is no longer respected by postgres process, for example:
```
ybd release --cxx-test pgwrapper_pg_analyze-test --gtest_filter PgAnalyzeTest.AnalyzeSamplingColocated --test-args '--vmodule=pg_sample=1' -n 2 -- -p 1 -k
zgrep pg_sample ~/logs/latest_test/1.log
```
shows no vlogs.

The reason is that `VLOG(1)` is used early by
```
#0  0x00007f7e1b48b090 in google::InitVLOG3__(google::SiteFlag*, int*, char const*, int)@plt () from /net/dev-server-timur/share/code/yugabyte-db/build/debug-clang19-dynamic-ninja/lib/libyb_util_shmem.so
#1  0x00007f7e1b47616e in yb::(anonymous namespace)::NegotiatorSharedState::WaitProposal (this=0x7f7e215e8000) at ../../src/yb/util/shmem/reserved_address_segment.cc:108
#2  0x00007f7e1b4781e0 in yb::AddressSegmentNegotiator::Impl::NegotiateChild (fd=45) at ../../src/yb/util/shmem/reserved_address_segment.cc:252
#3  0x00007f7e1b4737ce in yb::AddressSegmentNegotiator::NegotiateChild (fd=45) at ../../src/yb/util/shmem/reserved_address_segment.cc:376
#4  0x00007f7e1b742b7b in yb::tserver::SharedMemoryManager::InitializePostmaster (this=0x7f7e202e9788 <yb::pggate::PgSharedMemoryManager()::shared_mem_manager>, fd=45) at ../../src/yb/tserver/tserver_shared_mem.cc:252
#5  0x00007f7e2023588f in yb::pggate::PgSetupSharedMemoryAddressSegment () at ../../src/yb/yql/pggate/pg_shared_mem.cc:29
#6  0x00007f7e202788e9 in YBCSetupSharedMemoryAddressSegment () at ../../src/yb/yql/pggate/ybc_pg_shared_mem.cc:22
#7  0x000055636b8956f5 in PostmasterMain (argc=21, argv=0x52937fe4e790) at ../../../../../../src/postgres/src/backend/postmaster/postmaster.c:1083
#8  0x000055636b774bfe in PostgresServerProcessMain (argc=21, argv=0x52937fe4e790) at ../../../../../../src/postgres/src/backend/main/main.c:209
#9  0x000055636b7751f2 in main ()
```
and caches `vmodule` value before `InitGFlags` sets it from environment.

The fix is to explicitly call `UpdateVmodule` from `InitGFlags` after setting `vmodule`.
Jira: DB-15888

Test Plan:
```
ybd release --cxx-test pgwrapper_pg_analyze-test --gtest_filter PgAnalyzeTest.AnalyzeSamplingColocated --test-args '--vmodule=pg_sample=1' -n 2 -- -p 1 -k
zgrep pg_sample ~/logs/latest_test/1.log
```

Reviewers: hsunder

Reviewed By: hsunder

Subscribers: ybase, yql

Tags: #jenkins-ready, #jenkins-trigger

Differential Revision: https://phorge.dev.yugabyte.com/D42731

Author: ttyusupov

src/yb/util/flags.h

@@ -101,6 +101,8 @@ namespace yb {
 // See gflags.h for more information.
 void ParseCommandLineFlags(int* argc, char*** argv, bool remove_flags);
 
+void RegisterGlobalFlagsCallbacksOnce();
+
 // Reads the given file and updates the value of all flags specified in the file. Returns true on
 // success, false otherwise.
 bool RefreshFlagsFile(const std::string& filename);

src/yb/util/flags/flags.cc

@@ -642,7 +642,7 @@ Status LoadFlagsAllowlist() {
 
 }  // anonymous namespace
 
-void ParseCommandLineFlags(int* argc, char*** argv, bool remove_flags) {
+void RegisterGlobalFlagsCallbacksOnce() {
   static GoogleOnceType once_register_vmodule_callback = GOOGLE_ONCE_INIT;
   // We cannot use DEFINE_validator and REGISTER_CALLBACK for vmodule since it is not DEFINED in any
   // yb file, and we cannot guarantee the static initialization order. Instead we register them
@@ -653,7 +653,10 @@ void ParseCommandLineFlags(int* argc, char*** argv, bool remove_flags) {
     flags_callback_internal::RegisterGlobalFlagUpdateCallback(
         &FLAGS_vmodule, "ValidateAndUpdateVmodule", &UpdateVmodule);
   });
+}
 
+void ParseCommandLineFlags(int* argc, char*** argv, bool remove_flags) {
+  RegisterGlobalFlagsCallbacksOnce();
   CHECK_OK(LoadFlagsAllowlist());
 
   {

src/yb/yql/pggate/util/ybc_util.cc

@@ -96,14 +96,20 @@ Status InitGFlags(const char* argv0) {
     ChangeWorkingDir(pg_working_dir);
   });
 
+  // This is needed in case VLOG/VLOG_IS_ON was called before that and initial vmodule flag
+  // value has already been cached.
+  RegisterGlobalFlagsCallbacksOnce();
+
   // Also allow overriding flags on the command line using the appropriate environment variables.
   std::vector<google::CommandLineFlagInfo> flag_infos;
   google::GetAllFlags(&flag_infos);
   for (auto& flag_info : flag_infos) {
     string env_var_name = "FLAGS_" + flag_info.name;
     const char* env_var_value = getenv(env_var_name.c_str());
     if (env_var_value) {
-      google::SetCommandLineOption(flag_info.name.c_str(), env_var_value);
+      // Make sure callbacks are called.
+      RETURN_NOT_OK(flags_internal::SetFlagInternal(
+          flag_info.flag_ptr, flag_info.name.c_str(), env_var_value));
     }
   }