Fix build with C++20.

OpenTelemetry SDK built with C++20 uses std::span in the API, and we
want to match the standard to avoid type changes at the API boundary.

However, std::span constructor is explicit for non-default Extent,
requiring us to use a more verbose specification instead of an
initializer list.

Author: bavshin-f5

src/http_module.cpp

@@ -859,6 +859,8 @@ template <class Id>
 ngx_int_t hexIdVar(ngx_http_request_t* r, ngx_http_variable_value_t* v,
     uintptr_t data)
 {
+    namespace nostd = opentelemetry::nostd;
+
     auto ctx = ensureOtelCtx(r);
     if (!ctx) {
         return NGX_ERROR;
@@ -867,13 +869,13 @@ ngx_int_t hexIdVar(ngx_http_request_t* r, ngx_http_variable_value_t* v,
     auto id = (Id*)((char*)ctx + data);
 
     if (id->IsValid()) {
-        auto size = id->Id().size() * 2;
+        constexpr auto size = 2 * Id::kSize;
         auto buf = (char*)ngx_pnalloc(r->pool, size);
         if (buf == NULL) {
             return NGX_ERROR;
         }
 
-        id->ToLowerBase16({buf, size});
+        id->ToLowerBase16(nostd::span<char, size>{buf, size});
 
         v->len = size;
         v->valid = 1;

src/trace_context.hpp

@@ -67,16 +67,19 @@ struct TraceContext {
     static void serialize(const TraceContext& tc, char* out)
     {
         using namespace opentelemetry::trace::propagation;
+        namespace nostd = opentelemetry::nostd;
 
         *out++ = '0';
         *out++ = '0';
         *out++ = '-';
 
-        tc.traceId.ToLowerBase16({out, kTraceIdSize});
+        tc.traceId.ToLowerBase16(
+            nostd::span<char, kTraceIdSize>{out, kTraceIdSize});
         out += kTraceIdSize;
         *out++ = '-';
 
-        tc.spanId.ToLowerBase16({out, kSpanIdSize});
+        tc.spanId.ToLowerBase16(
+            nostd::span<char, kSpanIdSize>{out, kSpanIdSize});
         out += kSpanIdSize;
         *out++ = '-';