Exemplars not working

[mumrah]

I am not able to get exemplars working in version 0.17.0. Here is a basic reproduction:

from prometheus_client import start_http_server, Summary, Counter
import random
import time

# Create a metric to track time spent and requests made.
REQUEST_TIME = Summary('request_processing_seconds', 'Time spent processing request')
COUNTER = Counter("sample_count", "Sample counter")

# Decorate function with metric.
@REQUEST_TIME.time()
def process_request(t):
    """A dummy function that takes some time."""
    COUNTER.inc(1, exemplar={"foo": "bar"})
    time.sleep(t)

if __name__ == '__main__':
    # Start up the server to expose the metrics.
    start_http_server(8000)
    # Generate some requests.
    while True:
        process_request(random.random())

# HELP python_gc_objects_collected_total Objects collected during gc
# TYPE python_gc_objects_collected_total counter
python_gc_objects_collected_total{generation="0"} 246.0
python_gc_objects_collected_total{generation="1"} 114.0
python_gc_objects_collected_total{generation="2"} 0.0
# HELP python_gc_objects_uncollectable_total Uncollectable objects found during GC
# TYPE python_gc_objects_uncollectable_total counter
python_gc_objects_uncollectable_total{generation="0"} 0.0
python_gc_objects_uncollectable_total{generation="1"} 0.0
python_gc_objects_uncollectable_total{generation="2"} 0.0
# HELP python_gc_collections_total Number of times this generation was collected
# TYPE python_gc_collections_total counter
python_gc_collections_total{generation="0"} 39.0
python_gc_collections_total{generation="1"} 3.0
python_gc_collections_total{generation="2"} 0.0
# HELP python_info Python platform information
# TYPE python_info gauge
python_info{implementation="CPython",major="3",minor="9",patchlevel="16",version="3.9.16"} 1.0
# HELP request_processing_seconds Time spent processing request
# TYPE request_processing_seconds summary
request_processing_seconds_count 17.0
request_processing_seconds_sum 6.808643133
# HELP request_processing_seconds_created Time spent processing request
# TYPE request_processing_seconds_created gauge
request_processing_seconds_created 1.688505443345786e+09
# HELP sample_count_total Sample counter
# TYPE sample_count_total counter
sample_count_total 18.0
# HELP sample_count_created Sample counter
# TYPE sample_count_created gauge
sample_count_created 1.6885054433458078e+09

I also tried the text exporter and had the same result.

[mumrah]

I tried something like this

diff --git a/prometheus_client/exposition.py b/prometheus_client/exposition.py
index deaa6ed..b1f77ce 100644
--- a/prometheus_client/exposition.py
+++ b/prometheus_client/exposition.py
@@ -192,7 +192,11 @@ def generate_latest(registry: CollectorRegistry = REGISTRY) -> bytes:
         if line.timestamp is not None:
             # Convert to milliseconds.
             timestamp = f' {int(float(line.timestamp) * 1000):d}'
-        return f'{line.name}{labelstr} {floatToGoString(line.value)}{timestamp}\n'
+
+        exemplar = ''
+        if line.exemplar:
+            exemplar = ' # {' + ','.join(f'{key}="{value}"' for key, value in line.exemplar.labels.items()) + '}'
+        return f'{line.name}{labelstr} {floatToGoString(line.value)}{timestamp}{exemplar}\n'
 
     output = []
     for metric in registry.collect():

and it seemed to work. No clue if this is the right way to fix this though..

[csmarchbanks]

Hello, exemplars are only supported in the OpenMetrics exposition format, not in the basic Prometheus exposition format. If you add the header Accept: application/openmetrics-text to your request do you see exemplars?

Note that Prometheus will automatically negotiate to use OpenMetrics.

[mumrah]

Ah, thanks! After restarting Prometheus with --enable-feature=exemplar-storage I started seeing my exemplars show up.

Maybe a sentence clarifying this in the "Exemplars" section of the README would help other newbies like me :)