INFOPLAT-1575 Implement Prometheus to OTel metrics conversion and export

[pkcll]

Implement Prometheus to OTel metrics conversion and export.

Receiver:

• Wraps prometheusreceiver
• Fetches prometheus metrics data via prometheus.Gatherer (same process memory, no HTTP calls)
• Uses custom implementation of prometheus.scraper (from here INFOPLAT-1575 Implement metric scraper from prometheus.Gatherer  pkcll/prometheus#1) to shortcut HTTP request calls and fetch data from prometheus.Gatherer
• Converts Prometheus metrics into OTel format using prometheusreceiver
• Passes OTel metrics data to downstream OTel otlpexporter

Exporter:

• Wraps otlpexporter
• Receives metric data from the receiver
• Export OTel metrics data to otel collector endpoint via otlpexporter

Depends on

• INFOPLAT-1575 Implement metric scraper from prometheus.Gatherer INFOPLAT-1575 Implement metric scraper from prometheus.Gatherer  pkcll/prometheus#1

Run Example

cd chainlink-common/pkg/promotel

# start otel-collector 
docker run \
  -p 127.0.0.1:4317:4317 \
  -p 127.0.0.1:4318:4318 \
  -p 127.0.0.1:55679:55679 \
  otel/opentelemetry-collector-contrib:0.117.0

# run example
make run-example

Example app output

...
   INFO    cmd/example.go:34       Reported Prometheus metric      {"name": "test_counter_metric", "value": 7}
   INFO    cmd/example.go:48       Received Prometheus metric      {"name": "test_counter_metric", "value": 7}
   INFO    cmd/example.go:138      Exporting OTel metric   {"name": "test_counter_metric", "value": 7}
...

otel-collector-agent logs

otelcollectoragent  | Metric #27
otelcollectoragent  | Descriptor:
otelcollectoragent  |      -> Name: test_counter_metric
otelcollectoragent  |      -> Description:
otelcollectoragent  |      -> Unit:
otelcollectoragent  |      -> DataType: Sum
otelcollectoragent  |      -> IsMonotonic: true
otelcollectoragent  |      -> AggregationTemporality: Cumulative
otelcollectoragent  | NumberDataPoints #0
otelcollectoragent  | Data point attributes:
otelcollectoragent  |      -> app: Str(promotel-demo)
otelcollectoragent  | StartTimestamp: ...
otelcollectoragent  | Timestamp: ...
otelcollectoragent  | Value: 7.000000

[jmank88]

Why does this set a package global?