Prometheus exporter target_info key-value pairs must be string

[kornerc]

Describe your environment
...

Steps to reproduce
Prometheus expects labels to be string key-value pairs.
Currently, the labels are not cast to string for the target_info

Here is a minimal example which causes this problem:

import time

from prometheus_client import start_http_server

from opentelemetry import metrics
from opentelemetry.exporter.prometheus import PrometheusMetricReader
from opentelemetry.sdk.metrics import MeterProvider
from opentelemetry.sdk.resources import SERVICE_NAME, Resource

resource = Resource(attributes={
    SERVICE_NAME: "your-service-name",
    "foo": "bar",
    "hist": 1,
})

start_http_server(port=8000, addr="localhost")
reader = PrometheusMetricReader()
provider = MeterProvider(resource=resource, metric_readers=[reader])
metrics.set_meter_provider(provider)

meter = metrics.get_meter("my.meter.name")

work_counter = meter.create_counter(
    "work.counter", unit="1", description="Counts the amount of work done"
)

while True:
    work_counter.add(1, {"work.type": "Debug"})
    print("doing some work...")
    time.sleep(1)

It raises an exception in the Prometheus client: AttributeError: ("'int' object has no attribute 'replace'", Metric(target, Target metadata, info ... here prometheus_client\openmetrics\exposition.py", line 33, in <listcomp> k, v.replace('\\', r'\\').replace('\n', r'\n').replace('"', r'\"'))

What is the expected behavior?
Find a string representation for the target_info labels as it is done for the other metrics.

What is the actual behavior?
The target_info labels are exported in their original representation (non-strings).

Additional context
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