config: change NewOAuth2RoundTripper to accept variadic HTTPClientOption

[alliasgher]

Summary

NewOAuth2RoundTripper previously accepted *httpClientOptions as its fourth
parameter — an unexported type. This made the function effectively
uncallable with a non-nil options argument from any package outside
prometheus/common/config, forcing downstream users (e.g. Grafana Alloy) to
resort to reflection or unsafe pointer tricks.

Reported upstream in prometheus/prometheus#18329.

Change

Replace the unexported *httpClientOptions parameter with variadic
...HTTPClientOption — the exported interface already used by
NewRoundTripperFromConfigWithContext:

// Before (unusable externally with non-nil opts):
func NewOAuth2RoundTripper(cred SecretReader, cfg *OAuth2, next http.RoundTripper, opts *httpClientOptions) http.RoundTripper

// After (fully usable from any package):
func NewOAuth2RoundTripper(cred SecretReader, cfg *OAuth2, next http.RoundTripper, optFuncs ...HTTPClientOption) http.RoundTripper

Internally the function builds its own httpClientOptions copy from the supplied options starting from defaultHTTPClientOptions, so behaviour is identical to before when no options are passed.

Call sites updated

• Internal call in NewRoundTripperFromConfigWithContext: passes optFuncs... instead of &opts
• Three test call sites that passed &defaultHTTPClientOptions now pass no options (equivalent — defaults are unchanged)

Tests

go test ./config/...   # all pass

[roidelapluie]

Thanks!