message: Fix metric names in the manual
diff: @@ -435,17 +435,17 @@ Base Metrics
  The actor system collects this set of metrics always by default.
  -caf.running-actors
  +caf.system.running-actors
  - Tracks the current number of running actors in the system.
  - **Type**: ``int_gauge``
  - **Label dimensions**: none.
  -caf.processed-messages
  +caf.system.processed-messages
  - Counts the total number of processed messages.
  - **Type**: ``int_counter``
  - **Label dimensions**: none.
  -caf.rejected-messages
  +caf.system.rejected-messages
  - Counts the number of messages that where rejected because the target mailbox
  was closed or did not exist.
  - **Type**: ``int_counter``
  @@ -499,29 +499,29 @@ The configuration above would select all actors with names that start with
  For all actors that are selected by the user-defined filters, CAF collects this
  set of metrics:
  -caf.processing-time
  +caf.actor.processing-time
  - Samples how long the actor needs to process messages.
  - **Type**: ``dbl_histogram``
  - **Unit**: ``seconds``
  - **Label dimensions**: name.
  -caf.mailbox-time
  +caf.actor.mailbox-time
  - Samples how long messages wait in the mailbox before being processed.
  - **Type**: ``dbl_histogram``
  - **Unit**: ``seconds``
  - **Label dimensions**: name.
  -caf.mailbox-size
  +caf.actor.mailbox-size
  - Counts how many messages are currently waiting in the mailbox.
  - **Type**: ``int_gauge``
  - **Label dimensions**: name.
  -caf.stream.processed-elements
  +caf.actor.stream.processed-elements
  - Counts the total number of processed stream elements from upstream.
  - **Type**: ``int_counter``
  - **Label dimensions**: name, type.
  -caf.stream.input-buffer-size
  +caf.actor.stream.input-buffer-size
  - Tracks how many stream elements from upstream are currently buffered.
  - **Type**: ``int_gauge``
  - **Label dimensions**: name, type.
  