Add TLS support to prometheus_exporter

**Is your feature request related to a problem? Please describe.**

Provide support for using TLS when scraping the Prometheus endpoint.

**Describe the solution you'd like**
Similar to other output plugins, the usual security configuration for TLS:

```
[INPUT]
name fluentbit_metrics
tag internal_metrics
scrape_interval 2

[OUTPUT]
name prometheus_exporter
match internal_metrics
host 0.0.0.0
port 2021
tls on
tls.verify off
```

**Describe alternatives you've considered**

Prometheus remote write could be used, although currently needs to be enabled for Prometheus.
A secure tunnel could also be used.

**Additional context**

Securing the scraping of potentially sensitive cluster information.