[WIP] add and use tracing API as part of package logging

- make `logging.GetLogger(ctx, Subsys)` the authoritative `logger.Logger` factory function
    - the context carries a linked list of injected fields which
      `logging.GetLogger` adds to the logger it returns
- introduce the concept of tasks and spans, also tracked as linked list within ctx
    - [ ] TODO automatic logging of span begins and ends, with a unique
      ID stack that makes it easy to follow a series of log entries in
      concurrent code
    - ability to produce a chrome://tracing-compatible trace file,
      either via an env variable or a `zrepl pprof` subcommand
        - this is not a CPU profile, we already have go pprof for that
        - but it is very useful to visually inspect where the
          replication / snapshotter / pruner spends its time
          ( fixes #307 )

rpc/rpc_logging.go

@@ -3,17 +3,12 @@ package rpc
 import (
 	"context"
 
+	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/logger"
 )
 
 type Logger = logger.Logger
 
-type contextKey int
-
-const (
-	contextKeyLoggers contextKey = iota
-)
-
 /// All fields must be non-nil
 type Loggers struct {
 	General Logger
@@ -21,11 +16,10 @@ type Loggers struct {
 	Data    Logger
 }
 
-func WithLoggers(ctx context.Context, loggers Loggers) context.Context {
-	ctx = context.WithValue(ctx, contextKeyLoggers, loggers)
-	return ctx
-}
-
 func GetLoggersOrPanic(ctx context.Context) Loggers {
-	return ctx.Value(contextKeyLoggers).(Loggers)
+	return Loggers{
+		General: logging.GetLogger(ctx, logging.SubsysRPC),
+		Control: logging.GetLogger(ctx, logging.SubsysRPCControl),
+		Data:    logging.GetLogger(ctx, logging.SubsysRPCData),
+	}
 }