Revert to requesting Prometheus format by default

[alopezz]

What does this PR do?

It reverts the default accept header to request text/plain (i.e. prometheus format) that was introduced by #14445.

Motivation

It looks like the library support for OpenMetrics format generation and parsing is somewhat immature (e.g. prometheus/client_golang#829) which made setting the accept header to request the OpenMetrics format if available more disturbing than we anticipated.

Additional Notes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached
• If the PR doesn't need to be tested during QA, please add a qa/skip-qa label.

[codecov]

Codecov Report

Merging #15292 (ee52c5a) into master (f6e838f) will increase coverage by 0.36%.
The diff coverage is 100.00%.

Flag	Coverage Δ
active_directory	100.00% <ø> (+17.64%)	⬆️
activemq_xml	82.31% <ø> (ø)
airflow	90.00% <ø> (ø)
amazon_msk	89.07% <ø> (ø)
ambari	85.75% <ø> (ø)
apache	95.08% <ø> (ø)
arangodb	98.23% <ø> (ø)
argocd	88.04% <ø> (ø)
aspdotnet	100.00% <ø> (ø)
avi_vantage	92.54% <ø> (ø)
azure_iot_edge	82.08% <ø> (ø)
boundary	100.00% <ø> (ø)
btrfs	82.91% <ø> (ø)
cacti	87.90% <ø> (ø)
calico	84.41% <ø> (ø)
cassandra_nodetool	93.16% <ø> (ø)
ceph	91.07% <ø> (ø)
cert_manager	77.41% <ø> (ø)
cilium	80.45% <ø> (+0.90%)	⬆️
cisco_aci	95.27% <ø> (ø)
citrix_hypervisor	87.50% <ø> (ø)
clickhouse	95.49% <ø> (ø)
cloud_foundry_api	96.35% <ø> (+0.12%)	⬆️
cloudera	99.82% <ø> (ø)
cockroachdb	91.52% <ø> (ø)
consul	91.65% <ø> (ø)
coredns	94.57% <ø> (ø)
couch	95.43% <ø> (+0.24%)	⬆️
couchbase	84.28% <ø> (ø)
crio	89.79% <ø> (ø)
datadog_checks_base	89.62% <100.00%> (+0.34%)	⬆️
datadog_checks_dev	82.66% <ø> (+0.07%)	⬆️
datadog_checks_downloader	81.65% <ø> (ø)
datadog_cluster_agent	90.19% <ø> (ø)
dcgm	97.33% <ø> (ø)
directory	96.09% <ø> (+0.65%)	⬆️
disk	91.36% <ø> (ø)
dns_check	93.90% <ø> (ø)
dotnetclr	91.39% <ø> (+12.90%)	⬆️
druid	98.47% <ø> (ø)
ecs_fargate	83.06% <ø> (ø)
eks_fargate	94.05% <ø> (ø)
envoy	95.09% <ø> (+0.41%)	⬆️
etcd	95.56% <ø> (ø)
exchange_server	96.85% <ø> (+11.81%)	⬆️
external_dns	89.28% <ø> (ø)
fluentd	94.77% <ø> (ø)
foundationdb	78.50% <ø> (ø)
gearmand	78.26% <ø> (+1.24%)	⬆️
gitlab_runner	91.94% <ø> (ø)
glusterfs	80.09% <ø> (+0.92%)	⬆️
go_expvar	92.73% <ø> (ø)
gunicorn	92.85% <ø> (ø)
haproxy	95.13% <ø> (+0.16%)	⬆️
hazelcast	92.39% <ø> (ø)
hdfs_datanode	89.74% <ø> (ø)
hdfs_namenode	86.72% <ø> (ø)
http_check	96.09% <ø> (+2.15%)	⬆️
ibm_ace	91.79% <ø> (ø)
ibm_db2	95.30% <ø> (ø)
ibm_i	81.95% <ø> (ø)
ibm_mq	91.13% <ø> (-0.14%)	⬇️
ibm_was	96.08% <ø> (ø)
iis	95.00% <ø> (+37.60%)	⬆️
impala	97.97% <ø> (ø)
istio	77.43% <ø> (+0.55%)	⬆️
kafka_consumer	93.24% <ø> (ø)
kong	87.56% <ø> (ø)
kube_apiserver_metrics	97.85% <ø> (ø)
kube_controller_manager	96.00% <ø> (ø)
kube_dns	95.97% <ø> (ø)
kube_metrics_server	94.87% <ø> (ø)
kube_proxy	96.80% <ø> (ø)
kube_scheduler	96.53% <ø> (ø)
kubelet	90.99% <ø> (ø)
kubernetes_state	89.18% <ø> (ø)
kyototycoon	85.96% <ø> (ø)
lighttpd	83.64% <ø> (ø)
linkerd	85.14% <ø> (+1.14%)	⬆️
linux_proc_extras	96.22% <ø> (ø)
mapr	82.70% <ø> (ø)
mapreduce	81.81% <ø> (+0.46%)	⬆️
marathon	83.43% <ø> (ø)
mcache	93.50% <ø> (ø)
mesos_master	89.75% <ø> (ø)
mesos_slave	93.63% <ø> (ø)
mongo	96.55% <ø> (ø)
nagios	89.01% <ø> (ø)
network	94.01% <ø> (+1.10%)	⬆️
nfsstat	95.20% <ø> (ø)
nginx	95.24% <ø> (+0.54%)	⬆️
nginx_ingress_controller	98.36% <ø> (ø)
openldap	96.79% <ø> (ø)
openmetrics	98.08% <100.00%> (ø)
openstack	51.45% <ø> (ø)
openstack_controller	91.12% <ø> (ø)
oracle	89.78% <ø> (ø)
pdh_check	97.82% <ø> (ø)
pgbouncer	91.33% <ø> (ø)
php_fpm	90.25% <ø> (+0.84%)	⬆️
postfix	88.04% <ø> (ø)
powerdns_recursor	96.65% <ø> (ø)
process	85.42% <ø> (+0.28%)	⬆️
prometheus	94.17% <ø> (ø)
proxysql	98.97% <ø> (ø)
pulsar	100.00% <ø> (ø)
redisdb	87.78% <ø> (ø)
rethinkdb	97.93% <ø> (ø)
riak	99.22% <ø> (ø)
riakcs	93.61% <ø> (ø)
scylla	100.00% <ø> (ø)
silk	93.82% <ø> (ø)
singlestore	90.81% <ø> (ø)
snowflake	96.61% <ø> (ø)
spark	93.63% <ø> (ø)
squid	100.00% <ø> (ø)
ssh_check	91.58% <ø> (ø)
statsd	87.36% <ø> (+1.05%)	⬆️
strimzi	89.70% <ø> (ø)
supervisord	92.69% <ø> (ø)
system_core	90.90% <ø> (ø)
system_swap	98.30% <ø> (ø)
tcp_check	92.92% <ø> (ø)
teamcity	88.74% <ø> (+3.21%)	⬆️
temporal	100.00% <ø> (ø)
teradata	94.24% <ø> (ø)
tls	92.18% <ø> (+0.82%)	⬆️
tokumx	58.40% <ø> (ø)
torchserve	97.33% <ø> (ø)
traffic_server	96.13% <ø> (ø)
twemproxy	79.45% <ø> (ø)
twistlock	79.62% <ø> (ø)
varnish	84.39% <ø> (+0.26%)	⬆️
vault	95.53% <ø> (+0.57%)	⬆️
vertica	98.50% <ø> (ø)
voltdb	96.85% <ø> (ø)
vsphere	95.35% <ø> (+0.06%)	⬆️
weaviate	76.27% <ø> (ø)
win32_event_log	86.68% <ø> (+0.55%)	⬆️
windows_performance_counters	98.36% <ø> (ø)
windows_service	98.00% <ø> (ø)
wmi_check	92.91% <ø> (ø)
yarn	89.50% <ø> (ø)
zk	82.62% <ø> (+1.33%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

[github-actions]

Test Results

     973 files       973 suites   6h 52m 45s ⏱️
  5 715 tests   5 629 ✔️      75 💤 7 ❌ 4 🔥
23 178 runs  19 312 ✔️ 3 855 💤 7 ❌ 4 🔥

For more details on these failures and errors, see this check.

Results for commit ee52c5a.

[vivek-datadog]

Thanks Alex for the extensive background work done to conclude on the revert till a long term solution is identified 👍