drpc: add Logger interface replacing Log callback and drpcdebug

[cthumuluru-crdb]

Motivation

DRPC's logging has two problems:

1. Options.Log func(error) is an error-only callback with no log-level differentiation, requiring nil-checks at every callsite.
2. drpcdebug package provides build-tag-gated debug logging (-tags=debug) but is effectively dead code — no CI coverage, no tests, no active consumer. Last substantive change was in 2021.

This PR replaces both with a structured Logger interface (inspired by Pebble's logging pattern) and a new compile-time debug guard.

Changes

drpc.Logger interface

A four-method interface (Debugf, Infof, Errorf, Fatalf) in the top-level drpc package, alongside Handler, Conn, Transport, etc.

• DefaultLogger — Debugf is a no-op; other levels log to stderr.
  Suitable for production out of the box.
• InMemLogger — captures all output in memory for use in tests.

drpcserver: replace Log callback

Options.Log func(error) is replaced with Options.Logger. This is a breaking change — CockroachDB is the only consumer and is updated in a companion PR.

Logger propagation

Logger is threaded through Options in every package that emits log messages:

drpcserver → drpcmanager → drpcstream
                         → drpcwire.Writer
drpcclient (via DialOption) → drpcconn → drpcmanager → ...
drpcpool

Each constructor defaults opts.Logger to drpc.DefaultLogger if nil. No package stores a redundant copy — all access goes through opts.Logger.

drpcdebug package removed

The dead drpcdebug package is deleted. Its build-tag guard concept is preserved in a simpler form (see below).

Build-tag-gated debug logging

Debug log callsites sit on hot RPC paths (stream lifecycle, buffer flushes, packet reads). To guarantee zero cost in production, debug logging uses two layers of protection:

1. Compile-time constant — drpc.DebugEnabled is a const bool defined in a pair of build-tagged files (debug_on.go / debug_off.go). When the constant is false, the compiler eliminates guarded blocks entirely.

2. Callback pattern — log helpers take cb func() string instead of pre-formatted strings. Expensive formatting (fmt.Sprintf, .String()) is deferred and only evaluated when debug logging is compiled in.

func (m *Manager) log(what string, cb func() string) {
    if drpc.DebugEnabled {
        m.opts.Logger.Debugf("%s %s %s", m.String(), what, cb())
    }
}

Build	DebugEnabled	Debug blocks	Callback allocation
go build	false	eliminated by compiler	none
go build -tags=drpcdebug	true	active, routed through Logger	evaluated on call

Infof, Errorf, and Fatalf are always active regardless of the build tag.

Running tests with debug logging

go test -tags=drpcdebug ./...

Companion PR

cockroachdb/cockroach#169813 — implements drpcLogger routing DRPC log output to CockroachDB's DEV channel and injects it into both server and client pool.