collector/textfile: Avoid inconsistent help-texts (prometheus#2962)

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: prometheus#2317

Signed-off-by: Pranshu Srivastava <rexagod@gmail.com>

collector/fixtures/textfile/metrics_merge_different_help.out

@@ -8,4 +8,4 @@ node_textfile_mtime_seconds{file="fixtures/textfile/metrics_merge_different_help
 node_textfile_mtime_seconds{file="fixtures/textfile/metrics_merge_different_help/b.prom"} 1
 # 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 0
+node_textfile_scrape_error 1

collector/textfile.go

@@ -193,6 +193,7 @@ func (c *textFileCollector) Update(ch chan<- prometheus.Metric) error {
 	var errored bool
 	var parsedFamilies []*dto.MetricFamily
 	metricsNamesToFiles := map[string][]string{}
+	metricsNamesToHelpTexts := map[string][2]string{}
 
 	paths, err := filepath.Glob(c.path)
 	if err != nil || len(paths) == 0 {
@@ -218,6 +219,23 @@ func (c *textFileCollector) Update(ch chan<- prometheus.Metric) error {
 			mtime, families, err := c.processFile(path, f.Name(), ch)
 
 			for _, mf := range families {
+				// Check for metrics with inconsistent help texts and take the first help text occurrence.
+				if helpTexts, seen := metricsNamesToHelpTexts[*mf.Name]; seen {
+					if mf.Help != nil && helpTexts[0] != *mf.Help || helpTexts[1] != "" {
+						metricsNamesToHelpTexts[*mf.Name] = [2]string{helpTexts[0], *mf.Help}
+						errored = true
+						level.Error(c.logger).Log("msg", "inconsistent metric help text",
+							"metric", *mf.Name,
+							"original_help_text", helpTexts[0],
+							"new_help_text", *mf.Help,
+							// Only the first file path will be recorded in case of two or more inconsistent help texts.
+							"file", metricsNamesToFiles[*mf.Name][0])
+						continue
+					}
+				}
+				if mf.Help != nil {
+					metricsNamesToHelpTexts[*mf.Name] = [2]string{*mf.Help}
+				}
 				metricsNamesToFiles[*mf.Name] = append(metricsNamesToFiles[*mf.Name], metricsFilePath)
 				parsedFamilies = append(parsedFamilies, mf)
 			}