Add metrics to rate limiting

[MartinquaXD]

Fixes #180

In order to see how often we need to use the back off strategy for paraswap we need to have some metrics for it.

Test Plan

Metrics after manual test

# HELP gp_v2_api_rate_limiter_rate_limited_requests Number of responses indicating a rate limiting error.
# TYPE gp_v2_api_rate_limiter_rate_limited_requests counter
gp_v2_api_rate_limiter_rate_limited_requests{endpoint="paraswap"} 16
# HELP gp_v2_api_rate_limiter_requests_dropped Number of requests dropped while being rate limited.
# TYPE gp_v2_api_rate_limiter_requests_dropped counter
gp_v2_api_rate_limiter_requests_dropped{endpoint="paraswap"} 18
# HELP gp_v2_api_rate_limiter_successful_requests Number of successful requests.
# TYPE gp_v2_api_rate_limiter_successful_requests counter
gp_v2_api_rate_limiter_successful_requests{endpoint="paraswap"} 5