[#307] add package trace, integrate it with logging, and adopt it throughout zrepl

package trace:

- introduce the concept of tasks and spans, tracked as linked list within ctx
    - see package-level docs for an overview of the concepts
    - **main feature 1**: unique stack of task and span IDs
        - makes it easy to follow a series of log entries in concurrent code
    - **main feature 2**: 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 )

usage in package daemon/logging:

- goal: every log entry should have a trace field with the ID stack from package trace

- 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
    - `logging.GetLogger` also uses package `trace` to get the
      task-and-span-stack and injects it into the returned logger's fields

daemon/prometheus.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/zrepl/zrepl/config"
 	"github.com/zrepl/zrepl/daemon/job"
+	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/endpoint"
 	"github.com/zrepl/zrepl/logger"
 	"github.com/zrepl/zrepl/rpc/dataconn/frameconn"
@@ -86,16 +87,19 @@ func (j *prometheusJob) Run(ctx context.Context) {
 }
 
 type prometheusJobOutlet struct {
-	jobName string
 }
 
 var _ logger.Outlet = prometheusJobOutlet{}
 
-func newPrometheusLogOutlet(jobName string) prometheusJobOutlet {
-	return prometheusJobOutlet{jobName}
+func newPrometheusLogOutlet() prometheusJobOutlet {
+	return prometheusJobOutlet{}
 }
 
 func (o prometheusJobOutlet) WriteEntry(entry logger.Entry) error {
-	prom.taskLogEntries.WithLabelValues(o.jobName, entry.Level.String()).Inc()
+	jobFieldVal, ok := entry.Fields[logging.JobField].(string)
+	if !ok {
+		jobFieldVal = "_nojobid"
+	}
+	prom.taskLogEntries.WithLabelValues(jobFieldVal, entry.Level.String()).Inc()
 	return nil
 }