out_prometheus_exporter: Add expire time to metrics in prometheus_exporter

[bdjjiiik]

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Add expire time tometrics in prometheus_exporter.
These improvements are related to the fact that when some data sources are hit, the last value of the metric remains on the graphs, which worsens the understanding of the situation

Testing
Before we can approve your change; please submit the following in a comment:

• Example configuration file for the change

[SERVICE]
    flush       1
    log_level   info

[INPUT]
    Name          dummy
    Dummy         {"log": "dummy"}
    tag           dummy

[INPUT]
    Name              tail
    Path              fluent.log
    tag               logs

[FILTER]
    name               log_to_metrics
    tag                metrics
    match              logs
    metric_mode        gauge
    metric_name        message
    metric_description last message
    value_field        log

[FILTER]
    name               log_to_metrics
    tag                metric
    match              dummy
    metric_mode        gauge
    metric_name        dummy-message
    metric_description last dummy-message
    value_field        log

[OUTPUT]
    name            prometheus_exporter
    match           metrics
    host            0.0.0.0
    port            2021
    ttl             10s

• [N/A] Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• Run local packaging test showing all targets (including any new ones) build.
• Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• Documentation required for this feature

Backporting

• Backport to latest stable release.

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

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[github-actions]

This PR is stale because it has been open 45 days with no activity. Remove stale label or comment or this will be closed in 10 days.