Add session metrics endpoint

[eddyashton]

Resolves #2522.

I opted to put this under a new /node/metrics rather than /*/api/metrics, because it is exclusively node-local. /api/metrics is as well, but due to an implementation quirk needs to exposed separately on each frontend (each counts only its own endpoints). Its an API minor version bump in any case (our first since #2589), and this way it only affects the /node API.

[jumaffre]

It might also be interesting to give stats on the number of requests that have been forwarded by this node?

[achamayou]

This definitely deserves a CHANGELOG entry in my opinion.

[eddyashton]

It might also be interesting to give stats on the number of requests that have been forwarded by this node?

That's a good idea, but could/should probably be done in the existing /api/metrics. We count failed and succeeded requests, and in the same code path we sometimes decide to forward so could easily count those as well.

[ghost]

session_metrics@25654 aka 20210517.3 vs main ewma over 20 builds from 25297 to 25644

Click to see table

build_id	build_number	sb_sgx_cft^	sb_sgx_cft_mem	sb_sgx_bft^	sb_sgx_bft_mem	sb_ws_sgx_cft^	sb_ws_sgx_cft_mem	sb_sig_sgx_cft^	sb_sig_sgx_cft_mem	tpcc_sgx_cft^	tpcc_sgx_cft_mem	tpcc_sgx_bft^	tpcc_sgx_bft_mem	ls_sgx_cft^	ls_sgx_cft_mem	ls_ws_sgx_cft^	ls_ws_sgx_cft_mem	ls_jwt_sgx_cft^	ls_jwt_sgx_cft_mem	ls_js_sgx_cft^	ls_js_sgx_cft_mem	ls_js_jwt_sgx_cft^	ls_js_jwt_sgx_cft_mem	CHAMP put (/s)^	CHAMP get (/s)^	ls_full_js_sgx_cft^	ls_full_js_sgx_cft_mem
25297	20210510.33	27117.7	1.27181e+07	14055.6	3.81461e+07	33630.4	1.35046e+07	4879.46	1.00967e+07	5972.34	9.21478e+07	3377.97	1.59781e+08	21105.9	1.76989e+07	28887.5	1.76989e+07	4050.28	1.50774e+07	1886.2	1.08831e+07	1372.25	8.52384e+06	1.33576e+06	3.66369e+07	nan	nan
25309	20210510.36	25784.5	1.27181e+07	14336.3	3.86704e+07	31507.5	1.32424e+07	4967.03	1.03588e+07	6327.91	9.31964e+07	3398.89	1.66597e+08	21100.7	1.74367e+07	29223	1.76989e+07	3763.36	1.53396e+07	1856.08	1.08831e+07	1422.33	7.73741e+06	1.32968e+06	3.61831e+07	nan	nan
25350	20210511.1	29169.5	1.32424e+07	13673.7	4.02433e+07	33069.7	1.32424e+07	4893.9	1.03588e+07	6379.52	9.10992e+07	3438.46	1.74985e+08	24227.2	1.76989e+07	29280.5	1.7961e+07	3911.43	1.48153e+07	1886.16	7.99955e+06	1378.32	7.99955e+06	1.32884e+06	3.67025e+07	nan	nan
25376	20210511.7	27093.3	1.32424e+07	13946.1	3.81461e+07	32431.5	1.29803e+07	4857.46	1.00967e+07	6246.84	9.21478e+07	2957.94	1.9884e+08	20531.8	1.71746e+07	27195.7	1.7961e+07	4033.51	1.4291e+07	1899.82	8.2617e+06	1403.84	8.2617e+06	1.33794e+06	3.61837e+07	nan	nan
25389	20210511.9	27383.2	1.29803e+07	13114.7	3.81461e+07	34147.9	1.32424e+07	4820.38	1.00967e+07	6334.16	9.16235e+07	2774.77	2.45764e+08	22939.2	1.76989e+07	28075	1.76989e+07	3888.03	1.45532e+07	1976.58	1.08831e+07	1398.96	8.2617e+06	1.3626e+06	3.56168e+07	nan	nan
25410	20210511.14	29226.1	1.32424e+07	13799.7	3.81461e+07	32792.6	1.35046e+07	4859.48	1.00967e+07	6396.29	9.24099e+07	3333.89	1.90452e+08	20681.2	1.74367e+07	27310.2	1.76989e+07	3868.5	1.45532e+07	1913.94	8.52384e+06	1322.49	7.99955e+06	1.32598e+06	3.63114e+07	nan	nan
25421	20210511.16	28848.8	1.27181e+07	13736	3.70975e+07	34767.8	1.35046e+07	4952.56	1.00967e+07	6632.19	9.29342e+07	3387.72	1.68432e+08	21312	1.74367e+07	26357	1.76989e+07	4083.01	1.48153e+07	1947.14	8.2617e+06	1392.22	7.73741e+06	1.31458e+06	3.58042e+07	nan	nan
25426	20210511.18	25050.9	1.29803e+07	14643.4	3.81461e+07	34803.2	1.32424e+07	4910.89	1.08831e+07	6480.46	9.24099e+07	3070.88	2.16142e+08	23227.2	1.74367e+07	27224.8	1.76989e+07	4172.45	1.4291e+07	1912.49	1.14074e+07	1419.35	7.99955e+06	1.34727e+06	3.61199e+07	nan	nan
25430	20210512.1	28251.4	1.27181e+07	14022.8	3.89325e+07	32219.9	1.32424e+07	4910.66	1.03588e+07	6687.64	9.13613e+07	3033.28	2.0251e+08	23873.9	1.74367e+07	27017.9	1.76989e+07	3754.08	1.4291e+07	1936.31	8.52384e+06	1399.71	7.73741e+06	1.34691e+06	3.57417e+07	nan	nan
25449	20210512.7	25749.3	1.29803e+07	13904.6	7.11763e+07	30689.7	1.29803e+07	4944.8	1.03588e+07	6368.31	9.10992e+07	3401.91	1.7315e+08	22133.1	1.76989e+07	28357.2	1.76989e+07	3996.65	1.45532e+07	1875.8	1.11453e+07	1346.79	8.2617e+06	1.32299e+06	3.5993e+07	nan	nan
25474	20210512.15	28332.6	1.27181e+07	13675.5	3.86704e+07	33550.3	1.32424e+07	5111.19	1.03588e+07	6485.12	9.10992e+07	3398.1	1.67907e+08	23771.7	1.74367e+07	27178.9	1.76989e+07	3996.45	1.48153e+07	2251.2	1.19317e+07	1618.81	8.52384e+06	1.33532e+06	3.61831e+07	1927.44	1.08831e+07
25481	20210512.17	26392.7	1.27181e+07	13429.8	3.73597e+07	34006.3	1.35046e+07	4879.63	1.00967e+07	6734.17	9.08371e+07	3406.24	1.645e+08	21568.9	1.74367e+07	27486	1.76989e+07	4026.14	1.4291e+07	2271.92	9.04813e+06	1554.59	7.73741e+06	1.33759e+06	3.62471e+07	1939.43	8.2617e+06
25512	20210513.1	28432.9	1.29803e+07	14997.3	3.99811e+07	34376.6	1.35046e+07	4917	1.00967e+07	6505.25	9.26721e+07	3496.15	1.85733e+08	23151.8	1.74367e+07	27304	1.76989e+07	3787.35	1.4291e+07	2400.75	9.04813e+06	1611.69	8.52384e+06	1.35846e+06	3.69675e+07	1885.21	8.2617e+06
25549	20210513.13	24061.5	1.32424e+07	14542.1	3.81461e+07	32611.5	1.35046e+07	5046.02	1.0621e+07	6553.36	9.26721e+07	3287.29	1.91238e+08	22128.2	1.74367e+07	27973.3	1.76989e+07	4030.62	1.4291e+07	2249.91	8.78598e+06	1552.03	8.78598e+06	1.3316e+06	3.67025e+07	1890.99	8.2617e+06
25565	20210514.1	26189.2	1.29803e+07	14202.2	3.9719e+07	32198.8	1.32424e+07	4955.9	1.03588e+07	6582.32	9.29342e+07	2605.65	2.26103e+08	20883.4	1.71746e+07	27864.6	1.76989e+07	4029.01	1.45532e+07	2223.19	9.04813e+06	1609.81	8.2617e+06	1.35539e+06	3.62478e+07	1942.56	1.00967e+07
25575	20210514.4	29304.7	1.27181e+07	14549.7	3.89325e+07	31470	1.32424e+07	4829.83	1.03588e+07	6546.56	9.18856e+07	3373.4	1.58732e+08	21888.6	1.74367e+07	26974.9	1.76989e+07	4156.38	1.48153e+07	2276.48	9.04813e+06	1594.91	8.2617e+06	1.36907e+06	3.58036e+07	1911.44	1.0621e+07
25585	20210514.7	28315.6	1.27181e+07	13455.3	3.81461e+07	32956.5	1.35046e+07	4840.29	1.03588e+07	6679.91	9.21478e+07	3399.29	1.74461e+08	23451.7	1.76989e+07	28202.6	1.76989e+07	4232.1	1.48153e+07	2242.17	9.31027e+06	1558.96	8.52384e+06	1.33376e+06	3.58042e+07	1957.02	1.03588e+07
25598	20210514.11	27427.1	1.27181e+07	13767.2	3.73597e+07	33062.4	1.37667e+07	4899.32	1.0621e+07	6562.45	9.24099e+07	3382.02	1.70005e+08	22862.8	1.82232e+07	27341.1	1.74367e+07	3938.06	1.45532e+07	2239.45	9.04813e+06	1603.12	8.2617e+06	1.34339e+06	3.54939e+07	1974.9	1.11453e+07
25623	20210514.19	27630.3	1.27181e+07	14187.8	3.68354e+07	33408.6	1.32424e+07	4958.84	1.00967e+07	6788.99	9.26721e+07	3364.91	1.51917e+08	23522.8	1.71746e+07	29277.7	1.7961e+07	4111.21	1.45532e+07	2261.34	9.04813e+06	1491.54	7.99955e+06	1.32077e+06	3.62478e+07	1950.14	8.2617e+06
25644	20210517.1	29557.9	1.29803e+07	14712.9	4.05054e+07	32367	1.35046e+07	4904.8	1.03588e+07	6756.56	9.13613e+07	3398.63	1.57159e+08	21322.1	1.76989e+07	28212.5	1.7961e+07	4195.01	1.48153e+07	2241.7	9.04813e+06	1596.94	7.73741e+06	1.34861e+06	3.60557e+07	1940.38	1.00967e+07