docs(metrics): Correct code examples in markdown (aws-powertools#316)

Author: michaelbrewer

docs/core/metrics.md

@@ -114,7 +114,7 @@ This decorator also **validates**, **serializes**, and **flushes** all your metr
 
  @metrics.log_metrics
  def lambda_handler(evt, ctx):
-  metrics.add_metric(name="BookingConfirmation", unit="Count", value=1)
+  metrics.add_metric(name="BookingConfirmation", unit=MetricUnit.Count, value=1)
   ...
  ```
 === "Example CloudWatch Logs excerpt"
@@ -162,6 +162,8 @@ If you want to ensure that at least one metric is emitted, you can pass `raise_o
  ```python hl_lines="3"
  from aws_lambda_powertools.metrics import Metrics
 
+ metrics = Metrics()
+
  @metrics.log_metrics(raise_on_empty_metrics=True)
  def lambda_handler(evt, ctx):
   ...
@@ -183,12 +185,12 @@ When using multiple middlewares, use `log_metrics` as your **last decorator** wr
 tracer = Tracer(service="booking")
  metrics = Metrics(namespace="ExampleApplication", service="booking")
 
-metrics.add_metric(name="ColdStart", unit="Count", value=1)
+metrics.add_metric(name="ColdStart", unit=MetricUnit.Count, value=1)
 
  @metrics.log_metrics
  @tracer.capture_lambda_handler
  def lambda_handler(evt, ctx):
-  metrics.add_metric(name="BookingConfirmation", unit="Count", value=1)
+  metrics.add_metric(name="BookingConfirmation", unit=MetricUnit.Count, value=1)
   ...
  ```
 
@@ -200,7 +202,6 @@ You can optionally capture cold start metrics with `log_metrics` decorator via `
 
  ```python hl_lines="6"
  from aws_lambda_powertools import Metrics
- from aws_lambda_powertools.metrics import MetricUnit
 
  metrics = Metrics(service="ExampleService")
 
@@ -300,7 +301,7 @@ If you prefer not to use `log_metrics` because you might want to encapsulate add
  from aws_lambda_powertools.metrics import MetricUnit
 
  metrics = Metrics(namespace="ExampleApplication", service="booking")
- metrics.add_metric(name="ColdStart", unit="Count", value=1)
+ metrics.add_metric(name="ColdStart", unit=MetricUnit.Count, value=1)
 
  your_metrics_object = metrics.serialize_metric_set()
  metrics.clear_metrics()