rpc: add DRPC Logger integration routing to DEV channel

[cthumuluru-crdb]

Motivation

cockroachdb/drpc#57 adds a structured drpc.Logger interface to replace the old Log func(error) callback. This PR implements a CockroachDB-side logger that satisfies that interface, routing DRPC library log messages into CockroachDB's logging infrastructure.

Changes

drpcLogger implementation

A drpcLogger struct in pkg/rpc that satisfies drpc.Logger and routes all output to the DEV logging channel:

Method	CockroachDB routing	When visible
Debugf	log.Dev.VEventfDepth (verbosity 2)	--verbosity>=2 or --vmodule
Infof	log.Dev.InfofDepth	always
Errorf	log.Dev.ErrorfDepth	always
Fatalf	log.Dev.FatalfDepth	always

The logger carries a depth field so that *Depth calls report the correct DRPC callsite, not the logger wrapper.

Logger injection

The logger is injected at both entry points into the DRPC stack:

• NewDRPCServer — server-side, via drpcserver.Options.Logger
• DialDRPC — client-side, via drpcpool.Options.Logger

From there, DRPC propagates the logger internally through the manager, streams, and wire writer.

Linter registration

drpcLogger methods are registered in the fmtsafe linter allowlist (pkg/testutils/lint/passes/fmtsafe/functions.go) so the format-string safety checker recognizes them as printf-like functions.

Design decisions

• DEV channel (not Health or OPS) — matches the precedent set by grpcLogger in pkg/util/grpcutil/grpc_log.go. Library-internal logs go to DEV; RPC health events go to Health.
• Verbosity level 2 for Debugf — same threshold used for other verbose internal tracing. Keeps default logs clean while allowing operators to enable DRPC debug output for troubleshooting.

Debug logging is off by default

The DRPC drpcdebug build tag is not added to CockroachDB's default Bazel tags. Debug-level DRPC logs (buffer flushes, stream lifecycle, pool operations) are compiled out entirely — zero overhead on hot RPC paths.

To create a build with DRPC debug logging enabled:

./dev build cockroach --tags=drpcdebug

With a debug build, DRPC debug messages can be enabled at runtime via --verbosity=2 or --vmodule="drpc*=2".

Infof, Errorf, and Fatalf are always active regardless of the build tag — only Debugf callsites are gated.

Depends on

• cockroachdb/drpc#57 — adds the drpc.Logger interface, replaces Log callback, removes the dead drpcdebug package, adds compile-time debug guard.

Epic: none
Release note: None

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[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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[cockroach-teamcity]

This change is