Your recommended stack

2 areas to cover, based on what you told us

“We don't know how much traffic we can take”

1. Load & Performance Testing

Because you said: “We don't know how much traffic we can take”

Grafana k6Best fitfrom $19Free tier

Developer-first load testing: JavaScript scripts, Go engine, CI-native thresholds.

Why this fits you

Main limitation

The JavaScript runtime is not Node — no npm modules unless bundled, which surprises people.
Browser-level load testing exists but is much heavier and less mature than protocol-level testing.

The open-source workhorse: every protocol, a GUI, and twenty-five years of plugins.

Why this fits you

Main limitation

Thread-per-user model is memory-hungry; high load needs a cluster where k6 needs one box.
JMX test plans are XML that merges terribly in version control.

Gatlingfrom €89Free tier

High-performance load testing with a code-first DSL and superb HTML reports.

Why this fits you

Main limitation

Historically Scala-first; the Java and JavaScript DSLs are newer and less documented.
Fewer protocols than JMeter.

Because you said: “We don't know how much traffic we can take”

Synthetic checks correlated with APM traces, logs and infrastructure metrics.

Why this fits you

Correlation with APM traces and logs makes root-cause analysis dramatically faster.
Private locations let you monitor internal services behind the firewall.

Main limitation

Only worth it if you are already a Datadog customer; standalone it is expensive.
Datadog billing complexity is a well-known source of budget surprises.

Sentryfrom $26Free tier

Error tracking with stack traces, release attribution and now session replay.

Why this fits you

Release and commit attribution tells you which deploy introduced the error, and often who to ask.
Excellent SDK coverage across essentially every language and framework.

Main limitation

Event-based pricing punishes noisy applications until you invest in filtering.
The product has broadened a lot; some newer modules are shallower than the error core.