Avoid metrics with inconsistent help-texts. The earlier behaviour has
been preserved in the sense that the first encountered instance is still
used to generate metrics, whereas the subsequent inconsistent ones are
ignored along with a few peripheral changes.
```
# HELP node_scrape_collector_duration_seconds node_exporter: Duration of a collector scrape.
#TYPE node_scrape_collector_duration_seconds gauge
node_scrape_collector_duration_seconds{collector="textfile"} 0.0004005
# HELP node_scrape_collector_success node_exporter: Whether a collector succeeded.
# TYPE node_scrape_collector_success gauge
node_scrape_collector_success{collector="textfile"} 1
# HELP node_textfile_mtime_seconds Unixtime mtime of textfiles successfully read.
# TYPE node_textfile_mtime_seconds gauge
node_textfile_mtime_seconds{file="/Users/rexagod/repositories/misc/node_exporter/ne-bar.prom"} 1.710812009e+09
node_textfile_mtime_seconds{file="/Users/rexagod/repositories/misc/node_exporter/ne-foo.prom"} 1.710811982e+09
# HELP node_textfile_scrape_error 1 if there was an error opening or reading a file, 0 otherwise
# TYPE node_textfile_scrape_error gauge
node_textfile_scrape_error 1
# HELP promhttp_metric_handler_errors_total Total number of internal errors encountered by the promhttp metric handler.
# TYPE promhttp_metric_handler_errors_total counter
promhttp_metric_handler_errors_total{cause="encoding"} 0
promhttp_metric_handler_errors_total{cause="gathering"} 0
# HELP promhttp_metric_handler_requests_in_flight Current number of scrapes being served.
# TYPE promhttp_metric_handler_requests_in_flight gauge
promhttp_metric_handler_requests_in_flight 1
# HELP promhttp_metric_handler_requests_total Total number of scrapes by HTTP status code.
# TYPE promhttp_metric_handler_requests_total counter
promhttp_metric_handler_requests_total{code="200"} 0
promhttp_metric_handler_requests_total{code="500"} 0
promhttp_metric_handler_requests_total{code="503"} 0
# HELP tau_infrastructure_performing_maintenance_task At what timestamp a given task started or stopped, the last time it was run.
# TYPE tau_infrastructure_performing_maintenance_task gauge
tau_infrastructure_performing_maintenance_task{main_task="nightly",start_or_stop="start",sub_task="main"} 1.64728080198446e+09
```
Fixes: #2317
Signed-off-by: Pranshu Srivastava <rexagod@gmail.com>
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