[pkg/translator/prometheus] __ in metric name becomes _

[newly12]

Describe the bug
metric envoy__rule_engine_zlib_buf_error becomes envoy_rule_engine_zlib_buf_error after prometheus translator

Steps to reproduce
in this test, add following and execute

	require.Equal(t, "envoy__rule_engine_zlib_buf_error", CleanUpString("envoy__rule_engine_zlib_buf_error"))

        	Error:      	Not equal: 
        	            	expected: "envoy__rule_engine_zlib_buf_error"
        	            	actual  : "envoy_rule_engine_zlib_buf_error"

What did you expect to see?
envoy__rule_engine_zlib_buf_error

What did you see instead?
envoy_rule_engine_zlib_buf_error

What version did you use?
Version: latest commit

What config did you use?
Config: (e.g. the yaml config file)

Environment
OS: (e.g., "Ubuntu 20.04")
Compiler(if manually compiled): (e.g., "go 14.2")

Additional context
Add any other context about the problem here.