Fix async metrics test

[NelsonVides]

Often flushes happen either because of a timeout, or because of explicit
requests to flush the queue. Add an asynchronous wait for the metric to
change, and add a metric increment of explicit flush requests.

[codecov]

Codecov Report

Merging #3514 (717ef07) into master (fd9e1d2) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3514      +/-   ##
==========================================
+ Coverage   81.02%   81.06%   +0.04%     
==========================================
  Files         419      419              
  Lines       32309    32310       +1     
==========================================
+ Hits        26177    26191      +14     
+ Misses       6132     6119      -13     

Impacted Files	Coverage Δ
src/mongoose_batch_worker.erl	63.63% <100.00%> (+0.84%)	⬆️
src/http_upload/mod_http_upload.erl	94.62% <0.00%> (-2.16%)	⬇️
src/mod_muc_room.erl	76.97% <0.00%> (-0.12%)	⬇️
src/mod_muc_log.erl	78.11% <0.00%> (ø)
src/pubsub/mod_pubsub.erl	73.31% <0.00%> (+0.11%)	⬆️
src/ejabberd_c2s.erl	89.03% <0.00%> (+0.36%)	⬆️
src/domain/mongoose_domain_loader.erl	90.17% <0.00%> (+0.89%)	⬆️
src/mam/mod_mam_rdbms_arch_async.erl	97.77% <0.00%> (+1.11%)	⬆️
src/ejabberd_s2s_out.erl	60.98% <0.00%> (+1.86%)	⬆️

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[mongoose-im]

small_tests_24 / small_tests / 717ef07
Reports root / small

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small_tests_23 / small_tests / 717ef07
Reports root / small

---

dynamic_domains_pgsql_mnesia_24 / pgsql_mnesia / 717ef07
Reports root/ big
OK: 2691 / Failed: 0 / User-skipped: 238 / Auto-skipped: 0

---

dynamic_domains_mysql_redis_24 / mysql_redis / 717ef07
Reports root/ big
OK: 2674 / Failed: 0 / User-skipped: 255 / Auto-skipped: 0

---

dynamic_domains_mssql_mnesia_24 / odbc_mssql_mnesia / 717ef07
Reports root/ big
OK: 2691 / Failed: 0 / User-skipped: 238 / Auto-skipped: 0

---

dynamic_domains_pgsql_mnesia_23 / pgsql_mnesia / 717ef07
Reports root/ big
OK: 2691 / Failed: 0 / User-skipped: 238 / Auto-skipped: 0

---

ldap_mnesia_23 / ldap_mnesia / 717ef07
Reports root/ big
OK: 1500 / Failed: 0 / User-skipped: 390 / Auto-skipped: 0

---

ldap_mnesia_24 / ldap_mnesia / 717ef07
Reports root/ big
OK: 1500 / Failed: 0 / User-skipped: 390 / Auto-skipped: 0

---

internal_mnesia_24 / internal_mnesia / 717ef07
Reports root/ big
OK: 1541 / Failed: 0 / User-skipped: 349 / Auto-skipped: 0

---

pgsql_mnesia_24 / pgsql_mnesia / 717ef07
Reports root/ big
OK: 3078 / Failed: 0 / User-skipped: 247 / Auto-skipped: 0

---

pgsql_mnesia_23 / pgsql_mnesia / 717ef07
Reports root/ big
OK: 3078 / Failed: 0 / User-skipped: 247 / Auto-skipped: 0

---

elasticsearch_and_cassandra_24 / elasticsearch_and_cassandra_mnesia / 717ef07
Reports root/ big
OK: 1834 / Failed: 0 / User-skipped: 363 / Auto-skipped: 0

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mysql_redis_24 / mysql_redis / 717ef07
Reports root/ big
OK: 3073 / Failed: 0 / User-skipped: 252 / Auto-skipped: 0

---

mssql_mnesia_24 / odbc_mssql_mnesia / 717ef07
Reports root/ big
OK: 3078 / Failed: 0 / User-skipped: 247 / Auto-skipped: 0

---

riak_mnesia_24 / riak_mnesia / 717ef07
Reports root/ big
OK: 1680 / Failed: 0 / User-skipped: 363 / Auto-skipped: 0

[chrzaszcz]

Looks good, thanks for the fix!