Logging: structured slog messages throughout, and WithLogger options so callers choose the logger

Log messages are lower-case event names with their details as key/value
attributes ("error" for errors), in place of formatted strings.

Loggable package take a WithLogger option as part of their "New*" call,
defaulting to slog.Default(). Each tags its lines with its `component`.

Some debug loggers print directly rather than using slog when in
terminal logging mode.

internal/findatender/findatender.go

@@ -18,21 +18,45 @@ type FindATender struct {
 	config     *config.JobSearchConfig
 	apiRoot    string
 	ctx        context.Context
+	log        *slog.Logger
+}
+
+// Option configures a FindATender client.
+type Option func(*FindATender)
+
+// WithLogger makes the client log to logger instead of slog.Default().
+func WithLogger(logger *slog.Logger) Option {
+	return func(ft *FindATender) {
+		ft.log = logger
+	}
 }
 
 // New returns a client for the OCDS API at apiRoot. Requests time out after
-// 60 seconds and are retried once. Call Shutdown when finished with it.
-func New(conf *config.JobSearchConfig, apiRoot string) *FindATender {
+// 60 seconds and are retried once. Call Shutdown when finished with it. It
+// logs to slog.Default() unless given WithLogger.
+func New(conf *config.JobSearchConfig, apiRoot string, opts ...Option) *FindATender {
 	ft := &FindATender{
 		config:  conf,
 		apiRoot: apiRoot,
 	}
+	for _, opt := range opts {
+		opt(ft)
+	}
 	ft.httpClient = resty.New().
 		SetTimeout(60 * time.Duration(time.Second)).
 		SetRetryCount(1)
 	return ft
 }
 
+// logger returns the logger the client logs to, tagged with its component.
+func (ft *FindATender) logger() *slog.Logger {
+	log := ft.log
+	if log == nil {
+		log = slog.Default()
+	}
+	return log.With("component", "findatender")
+}
+
 // ReloadConfig replaces the configuration the client reads, such as the log
 // level. The HTTP client and API root set by New are not changed.
 func (ft *FindATender) ReloadConfig(conf *config.JobSearchConfig) {
@@ -55,7 +79,7 @@ func (ft *FindATender) With(ctx context.Context) *FindATender {
 // logged, as there is nothing a caller could do about it.
 func (ft *FindATender) Shutdown() {
 	if err := ft.httpClient.Close(); err != nil {
-		slog.Warn("findatender: closing HTTP client", "error", err)
+		ft.logger().Warn("closing http client failed", "error", err)
 	}
 }