refactor(bvar): expand metric constructor macro

[darion-yaphet]

Summary

• Remove the COMMON_VARIABLE_CONSTRUCTOR macro
• Replace its uses in the Babylon counter variants of Adder, Maxer, Miner, and IntRecorder with explicit constructors
• Preserve the existing constructor behavior for default construction, expose(name), and expose_as(prefix, name)

Motivation

COMMON_VARIABLE_CONSTRUCTOR hid public constructors behind macro expansion, making the code harder to read, search, and navigate with regular C++ tooling.

This change keeps the API behavior unchanged while making the constructors visible at their declaration sites.

using Base::Base was intentionally avoided because it would expose additional internal base constructors in the Babylon counter path and could broaden the public
API.

Testing

• cmake --build build --target brpc-static -j6
• git diff --check
• rg COMMON_VARIABLE_CONSTRUCTOR . -g '!build/**'

Notes

The WITH_BABYLON_COUNTER path was not compiled locally because the available Bazel setup is missing the repository-required Bazel 7.2.1 binary.

[chenBright]

I think this is to simplify the code, which is fine, like the DISALLOW_COPY_AND_ASSIGN macro.