Fix Prometheus import error when running Porter

Commit a4de8a88 introduced amazing changes and solved linting errors
like F821. To achive it, this change introduced the import of
PrometheusMetricsConfig in lawful.py, which is dependant of
prometheus-client package.

Ursula imports prometheus-client, so no problem with this. But Porter
does not. So a import error is raised when Porter is run.

Also, a remaining TODO comment has been cleaned. This TODO is part of a
larger comment and was introduced in commit 424e373a and solved
afterwards.

newsfragments/3202.bugfix.rst

@@ -0,0 +1 @@
+Fix Prometheus import error when running Porter

nucypher/characters/lawful.py

@@ -4,6 +4,7 @@ import time
 from pathlib import Path
 from queue import Queue
 from typing import (
+    TYPE_CHECKING,
     Any,
     Dict,
     Iterable,
@@ -116,7 +117,9 @@ from nucypher.policy.policies import Policy
 from nucypher.utilities.emitters import StdoutEmitter
 from nucypher.utilities.logging import Logger
 from nucypher.utilities.networking import validate_operator_ip
-from nucypher.utilities.prometheus.metrics import PrometheusMetricsConfig
+
+if TYPE_CHECKING:
+    from nucypher.utilities.prometheus.metrics import PrometheusMetricsConfig
 
 
 class Alice(Character, actors.PolicyAuthor):
@@ -990,13 +993,13 @@ class Ursula(Teacher, Character, actors.Operator, actors.Ritualist):
     def run(
         self,
         emitter: StdoutEmitter = None,
-        discovery: bool = True,  # TODO: see below
+        discovery: bool = True,
         availability: bool = False,
         worker: bool = True,
         ritualist: bool = True,
         hendrix: bool = True,
         start_reactor: bool = True,
-        prometheus_config: PrometheusMetricsConfig = None,
+        prometheus_config: "PrometheusMetricsConfig" = None,
         preflight: bool = True,
         block_until_ready: bool = True,
         eager: bool = False,