MINOR: stats: properly handle ST_F_CHECK_DURATION metric

ST_F_CHECK_DURATION metric is typed as unsigned int variable, and it is
derived from check->duration that is signed.

While most of the time check->duration > 0, it is not always true:
with HCHK_STATUS_HANA checks, check->duration is set to -1 to prevent server
logs from including irrelevant duration info (HCHK_STATUS_HANA checks are not
time related).

Because of this, stats could report UINT64_MAX value for ST_F_CHECK_DURATION
metric. This was quite confusing. To prevent this, we make sure not to assign
negative value to ST_F_CHECK_DURATION.

This is only a minor printing issue, not backport needed.

Author: Darlelet

src/stats.c

@@ -2330,7 +2330,7 @@ int stats_fill_sv_stats(struct proxy *px, struct server *sv, int flags,
 			case ST_F_CHECK_DURATION:
 				if ((sv->check.state & (CHK_ST_ENABLED|CHK_ST_PAUSED)) == CHK_ST_ENABLED &&
 					sv->check.status >= HCHK_STATUS_CHECKED)
-					metric = mkf_u64(FN_DURATION, sv->check.duration);
+					metric = mkf_u64(FN_DURATION, MAX(sv->check.duration, 0));
 				break;
 			case ST_F_CHECK_DESC:
 				if ((sv->check.state & (CHK_ST_ENABLED|CHK_ST_PAUSED)) == CHK_ST_ENABLED)