[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 )

cli/cli.go

@@ -1,6 +1,7 @@
 package cli
 
 import (
+	"context"
 	"fmt"
 	"os"
 
@@ -8,6 +9,7 @@ import (
 	"github.com/spf13/pflag"
 
 	"github.com/zrepl/zrepl/config"
+	"github.com/zrepl/zrepl/daemon/logging"
 )
 
 var rootArgs struct {
@@ -75,7 +77,7 @@ type Subcommand struct {
 	Short            string
 	Example          string
 	NoRequireConfig  bool
-	Run              func(subcommand *Subcommand, args []string) error
+	Run              func(ctx context.Context, subcommand *Subcommand, args []string) error
 	SetupFlags       func(f *pflag.FlagSet)
 	SetupSubcommands func() []*Subcommand
 
@@ -96,7 +98,11 @@ func (s *Subcommand) Config() *config.Config {
 
 func (s *Subcommand) run(cmd *cobra.Command, args []string) {
 	s.tryParseConfig()
-	err := s.Run(s, args)
+	ctx := context.Background()
+	endTask := logging.WithTaskFromStackUpdateCtx(&ctx)
+	defer endTask()
+	err := s.Run(ctx, s, args)
+	endTask()
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "%s\n", err)
 		os.Exit(1)