VM Statistics Collector

Collects Erlang VM metrics using :erlang.statistics/1.

Exported metrics

```
erlang_vm_statistics_bytes_output_total
```
Type: counter.
The total number of bytes output to ports.
```
erlang_vm_statistics_bytes_received_total
```
Type: counter.
The total number of bytes received through ports.
```
erlang_vm_statistics_context_switches
```
Type: counter.
The total number of context switches since the system started.
```
erlang_vm_statistics_garbage_collection_number_of_gcs
```
Type: counter.
The total number of garbage collections since the system started.
```
erlang_vm_statistics_garbage_collection_words_reclaimed
```
Type: counter.
The total number of words reclaimed by GC since the system started.
```
erlang_vm_statistics_garbage_collection_bytes_reclaimed
```
Type: counter.
The total number of bytes reclaimed by GC since the system started.
```
erlang_vm_statistics_reductions_total
```
Type: counter.
Total reductions count.
```
erlang_vm_statistics_run_queues_length_total
```
Type: gauge.
The total length of the run-queues. That is, the number of processes and ports that are ready to run on all available run-queues.
```
erlang_vm_statistics_runtime_milliseconds
```
Type: counter.
The sum of the runtime for all threads in the Erlang runtime system.
```
erlang_vm_statistics_wallclock_time_milliseconds
```
Type: counter.
Can be used in the same manner as erlang_vm_statistics_runtime_milliseconds, except that real time is measured as opposed to runtime or CPU time.

Metrics exported by this collector can be configured via :vm_statistics_collector_metrics key of :prometheus app environment.

Options are the same as Item parameter values for :erlang.statistics/1:

:context_switches for erlang_vm_statistics_context_switches;
:garbage_collection for erlang_vm_statistics_garbage_collection_number_of_gcs, :erlang_vm_statistics_garbage_collection_bytes_reclaimed, and :erlang_vm_statistics_garbage_collection_words_reclaimed;
:io for erlang_vm_statistics_bytes_output_total and erlang_vm_statistics_bytes_received_total;
:reductions for erlang_vm_statistics_reductions_total;
:run_queue for erlang_vm_statistics_run_queues_length_total;
:runtime for erlang_vm_statistics_runtime_milliseconds;
:wall_clock for erlang_vm_statistics_wallclock_time_milliseconds.

By default all metrics are enabled.